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IC 


9045 



Bureau of Mines Information Circular/1985 



Water-Jet-Assisted Cutting 

Proceedings: Bureau of Mines Open Industry 
Meeting, Pittsburgh, PA, June 21, 1984 



Compiled by Charles D. Taylor and Robert J. Evans 



<^- 




UNITED STATES DEPARTMENT OF THE INTERIOR 



tflNES 75TH A^ 



Information Circular 9045 

// 



Water-Jet-Assisted Cutting 

Proceedings: Bureau of Mines Open Industry 
Meeting, Pittsburgh, PA, June 21, 1984 



Compiled by Charles D. Taylor and Robert J. Evans 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




w 






XP- 



<\v 



Library of Congress Cataloging in Publication Data: 



Water-jet-assisted cutting 








(Information circular ; 


9045) 






Bibliography. 








Supt. of Docs, no.: I 2 


3.27:9045. 






1. Hydraulic mining- 


-Congresses. 


2. Water-jet— Congresses. 3. 


Jet cutting— Congresses. 


I. Taylor, 


Charles D. (Charles 


Darrell), 


1946- . [I. Evans, Robert J. III. United States. Bureau 


of Mines. 


IV. Scries: Information 


circular (United States. Bureau of 


Mines) ; 


9045. 








TN295.U4 [TN2781 


622s [622' 


.321 85-600095 





Go 

S CONTENTS 

Page 
O 

Abstract 1 

^_q Introduction, by John N. Murphy and Bradley V. Johnson 2 

"^\Water-jet-assisted rock cutting — the present state of the art, by Michael Hood. 3 
— Analysis of mechanical tool force reductions when using water-jet-assisted cut- 

' ting, by R. J. Evans, H. J. Handewith, and C. D. Taylor 21 

Experience with boom-type roadheaders equipped with high-pressure water-jet 
systems for roadway drivage in British coal mines, by A. H. Morris and 

M. G. Tomlin 29 

Development work for coal winning technology, by Dr. E. H. Henkel 40 

The water-jet plow, by David A. Summers 49 

Design review of Jarvis Clark jetbolter, by William C. Griffiths 58 

Water-jet-assisted tunnel boring, by Dr. Levent Ozdemir 63 

Investigation of optimizing traverse speed of water-jet-assisted drag picks, 

by R. J. Evans, H. J. Handewith, and C. D. Taylor 69 

Optimization of water-jet systems for mining applications, by 

Dr. James M. Reichman 77 



do 

o 

a) 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS 


REPORT 


cm 


centimeter 


L/min 


liter per minute 


ft 


foot 


lb 


pound 


ft/h 


foot per hour 


lb/ft 3 


pound per cubic foot 


ft-lb 


foot pound 


Ibf 


pound force 


\ ft/lbf 


foot pound force 


lbf/in 2 


pound force per 
square inch 


ft/mi n 


foot per minute 










m 


meter 


g/m 3 


gram per cubic meter 










m 3 /(kW-h) 


cubic meter per 


gal 


gallon 




kilowatt hour 


gal/min 


gallon per minute 


m/min 


meter per minute 


h 


hour 


m/s 


meter per second 


hp 


horsepower 


mg/m 3 


milligram per 
cubic meter 


Hz 


Hertz 










min 


minute 


in 


inch 










MJ/m 3 


megajoule per 


(in-lbf)/in 3 


energy/ volume (relative 
measure of cutting 




cubic meter 




efficiency) 


mm 


millimeter 


in/r 


inch per revolution 


mm/s 


millimeter per second 


in/s 


inch per second 


ym 


micrometer ! 


kg 


kilogram 


MN/m 


meganewton per meter 


kj 


kilo joule 


MPa 


megapascal 


kJ/m 


kilojoule per meter 


pet 


percent 


kJ/m 3 


kilojoule per cubic 
meter 


psi 


pound per square inch 






psig 


pound per square inch, 


km 


kilometer 




gauge 


kN 


kilonewton 


r/min 


revolution per minute 


ksi 


thousand pounds per 
square inch 


s 


second 






ton/h 


ton (short) per hour 


kW 


kilowatt 










ton/min 


ton (short) per minute 


kW-h 


kilowatt hour 










wt pet 


weight percent 


kW/m 2 


kilowatt per square 
meter 







WATER-JET-ASSISTED CUTTING 

Proceedings: Bureau of Mines Open Industry Meeting, 
Pittsburgh, PA, June 21, 1984 

Compiled by Charles D. Taylor and Robert J. Evans 



ABSTRACT 

Greater mining productivity requires a more efficient cutting process. 
The cutting force available from today's mining machines has been opti- 
mized with respect to machine size and weight. Researchers have shown 
that when employing water-jet-assisted cutting, bit forces and drum 
torques can be reduced significantly, which may allow mining machines to 
become lighter and more efficient. 

The Bureau of Mines has initiated a program to develop a water- 
jet-assisted rotary cutting system using the conventional bit assisted 
by a directed water jet operating at moderate pressures (3,000 to 
10,000 psi). This water-jet-assisted cutting system has the potential 
to improve cutting efficiency without increasing machine horsepower or 
water usage (beyond what is presently used for dust control) or requir- 
ing fundamental changes in mining practice. In- addition to improvements 
in productivity, other anticipated benefits of water-jet cutting include 
reduced generation of respirable dust, elimination of frictional igni- 
tions, increased bit life, reduced fines, and fewer machine vibrations. 

The papers presented at this open industry meeting discuss the devel- 
opment of water-jet-assisted cutting technology and future application 
of this technology to a variety of mining techniques including roof 
drilling and longwall mining. 



1 1ndustrial hygienist. 
^Civil engineer. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 
By John N. Murphy 1 and Bradley V. Johnson 2 



There have been numerous investigations 
relative to the use of high-pressure wa- 
ter jets to cut various materials; mate- 
rials considered for this type of cutting 
process have ranged from relatively soft 
materials, such as coal, to harder sub- 
stances, including granite and quartzitic 
materials. While there has been signifi- 
cant progress in this area and some nota- 
ble successes for specific applications, 
many problems remain, including general 
system performance, erosion of jets at 
high pressure, jet stability, and the 
availability and reliability of rotating 
high-pressure seals. 

Research conducted by the Bureau and 
by Bureau of Mines contractors has 
shown that the application of relatively 
low-pressure water jets, i.e., 5,000 to 
20,000 psi, in conjunction with the 
classical mechanical cutting pick, gives 
significant increases in performance. 
Performance has been measured in a vari- 
ety of ways, including reductions in the 
cutting forces required, the wear life 
of the mechanical cutter bit, reduced 
machine vibration, and reductions in gen- 
erated dust and noise. While these en- 
hancements are of direct benefit in terms 
of today's mining machinery, they offer 
the potential for the application of ex- 
isting machines to more difficult min- 
ing conditions, e.g., hard roof, or al- 
ternatively higher production rates in 
less difficult strata, as well as the 
health and safety benefits of reduced 

— i 

'Research director, Pittsburgh Research 

Center, Bureau of Mines, Pittsburgh, PA. 

2 Staff engineer, Division of Conserva- 
tion and Development, Mining Research, 
Bureau of Mines, Washington, DC. 



dust and noise. Perhaps equally important 
are the future considerations in terms of 
machine design realizing the benefits of 
water-jet-assisted cutting; specifically, 
smaller, lighter, more mobile machines 
can be realized. The significant reduc- 
tions in vibrations that have been ob- 
served to date also offer the opportunity 
for significant changes in the structural 
design of machines. 

It has also been demonstrated, in a 
preliminary fashion, that the application 
of the low-pressure water jets can sig- 
nificantly reduce, or perhaps eliminate, 
concerns about frictional ignition from 
cutting picks impacting hard roof or py- 
ritic inclusions. 

As the proceedings of this Bureau of 
Mines-sponsored water-jet-cutting seminar 
will demonstrate, significant progress 
has been made in the understanding of the 
water-jet-assisted cutting process, the 
optimization of machine design parame- 
ters, and the field trials of various 
prototype machines utilizing water-jet- 
assisted cutting. The results to date 
have been extremely promising, which in 
part was the basis for this seminar, so 
that the industry could be aware of and 
utilize the progress made to date. How- 
ever, more work is required before design 
engineers can successfully apply this 
promising technology to the machine of 
their choice. Ongoing Bureau work will 
certainly contribute toward the reali- 
zation of this goal. These proceedings 
provide a realistic assessment of the 
current state of the art, and the bib- 
liographic references with each article 
can provide additional information if 
required. 



WATER-JET-ASSISTED ROCK CUTTING — THE PRESENT STATE OF THE ART 

By Michael Hood 1 



ABSTRACT 



The benefits of using moderate-pressure 
water jets to assist mechanical tools, 
notably drag bits, are reviewed. These 
benefits include reduced bit forces, es- 
pecially the bit normal force; reduced 
bit wear; reduced dust make; and reduced 
incidence of frictional sparking. The 
research work that has been conducted to 
date to investigate this phenomenon has 
been largely empirical. Experiments are 
described that extend the data bank of 
this empirical knowledge. In addition, 
experiments aimed at gaining a better un- 
derstanding of the fundamentals of the 
rock fragmentation process with this hy- 
brid cutting method are outlined. 

Results from the first of these experi- 
mental series are used to make recommen- 
dations as to the position of the jet 
with respect to the bit, the standoff 
distance between the nozzle exit and the 
bit-rock interface, and the jet energy. 
In addition, preliminary findings are 



reported regarding the increased jet en- 
ergy necessary to maintain substantial 
reductions in the bit forces when the bit 
velocity is increased. Results from the 
second test series are discussed in the 
context of rock fracture behavior induced 
by mechanical tools acting alone. The 
likely influence of water jets on these 
fracture processes is analyzed. It is 
concluded that , in terms of the bit force 
reductions, a dominant effect of the jets 
when used in conjunction with sharp drag 
bits is continuous removal of the rock 
debris that forms ahead of the advancing 
bit. The observed reductions of bit wear 
and incidence of frictional sparking are 
attributed to reduced heat loading of the 
bit during the cutting operation. Reduc- 
tions in the dust make are attributed to 
effective wetting of the fine rock parti- 
cles before they become entrained in the 
airstream. 



INTRODUCTION 



Breaking rock from an in situ rock mass 
is carried out today using either a com- 
bination of mechanical tools (to drill 
holes) and explosives, or mechanical 
tools alone to cut the material from the 
face. It is evident that mechanical 
tools play a crucial role in what could 
be termed primary rock breaking, and it 
is instructive to review the princi- 
ples by which these tools induce rock 
fracture. 

First, the importance of rock breaking 
in modern western mining should be high- 
lighted. About 8.5 million tons of ore 
and waste are mined each year in the 
United States alone ( 1_) . 2 Tunnel drivage 

' Associate professor, Mining Engi- 
neering, University of California, at 
Berkeley, CA. 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this paper. 



each year by the countries of the Organi- 
zation of Economic Cooperation and Devel- 
opment (OECD) is about 50,000 km. In the 



10,000 km of 
year just in 

of the total 
in the United 
kW'h annually, 



United States alone about 
roadways are driven each 
coal mines. Almost 2 pet 
electric power generated 
States, about 34 billion 
is used in rock breakage processes. Ad- 
mittedly, most of this energy is spent in 
secondary rock breaking, i.e., crushing 
and comminution, rather than in primary 
breaking from an in situ rock mass. 
However, these secondary processes also 
employ mechanical means to induce frac- 
ture. It should be noted the rock break- 
ing processes are extremely inefficient. 
In primary processes, less than 10 pet of 
the energy supplied at the cutting bit 
generates new surface area, and in sec- 
ondary processes, less than 1 pet of the 
applied energy is used to generate new 
surfaces (1). Thus, a conclusion may be 



drawn that vast quantities of material 
are extracted from the earth's crust each 



year and that this activity 
large quantities of energy. 



utilizes 



ACKNOWLEDGMENT 



This work was funded by the U.S. De- 
partment of Energy under the Fossil Fuel, 



Coal Mining Program. Later this program 
was transferred to the Bureau of Mines. 



MECHANICALLY INDUCED ROCK FRACTURE PROCESSES 



A mechanical tool induces fracture in 
rock by the application of load through 
the tool to the rock. This load can be 
resolved into components acting normal 
and parallel to the rock surface. With 
some tools, for example, rolling cutters 
such as disc cutters and tricone bits, 
the normal force is the dominant compo- 
nent (fig. L4). With other tools, for 
example, sharp drag bits, the parallel or 
cutting force is the dominant component 
(fig. IS). The former may be described 
as an indentation process, and the latter 



A 



\7 



/ 



/K 



\ 



y/w/^/WA&y/< 



\ 



/ 



M 



MW/AMWAM* 



B 



c 




FIGURE 1. - Normal {A) and shearer (S) 
forces applied to induce rock fracture. 



as a cleavage process. Both of these 
techniques were used by stone age man. 

Figure 2A shows the stress distribution 
in an elastic halfspace beneath a circu- 
lar punch: the indentation case. Figure 
2B shows the stress distribution in an 
elastic quarterplane with a line load 
normal to one of the surfaces: the 
cleavage case. Despite the differences 
in the way these loads are applied, the 
final result in terms of the mechanisms 
of crack propagation appears to be the 
same for both indentation and cleavage 
loading systems. In both cases chip for- 
mation occurs as a result of tensile 
crack growth. Tensile cracking ahead of 



%i Edge of punch 




B 



Edge of punch 




FIGURE 2. - Stress distributions for in- 
dentation (.4) and cleavage (£>) cases. 



sharp drag bits was recognized by Evans 
and Murrell (2^) . More recent work (3_-4) 
has shown that, although the stress field 
induced in the rock by an indenter is 
predominantly compressive, a tensile 
crack is initiated adjacent to the cor- 
ners of the tool. This crack propagates 
initially in a Hertzian manner at some 
angle to the rock surface. Propagation 
of this crack then ceases, and the appli- 
cation of additional load results in 
failure by triaxial crushing beneath the 
tool. Subsequently a tensile crack, ini- 
tiated towards the base of the Hertzian 
crack, propagates up to the surface to 



form a rock chip. The rock chips formed 
by both indentation and cleavage process- 
es show a marked similarity, substantiat- 
ing the claim that the final mechanisms 
of failure, i.e., crack growth, are iden- 
tical. The relative inefficiency of the 
indentation process can be explained us- 
ing this model in terms of the additional 
energy required to crush the rock beneath 
the tool in order to propagate the crack 
that forms the rock chip. Early man rec- 
ognized that cleavage was the more effi- 
cient of the two breaking processes and 
used this method for forming arrowheads . 



CURRENT ROCK BREAKING PRACTICE 



The current state of the art is that 
most primary rock breaking is accom- 
plished by drilling and blasting. In 
hard and medium-strength rocks, drill 
bits apply indentation loads to the rock 
to induce fracture. Only in weak rocks 
are rotary drills that induce fracture 
by cleavage employed. The reason that 
cleavage is selected as a breaking method 
only in a few rock cutting situations is 
the limited strength of the bit materi- 
als. Cemented tungsten carbide, which is 
used ubiquitously as a cutting edge for 
rock bits, is a brittle material. It has 
extremely high strength in compression 
but is relatively weak in tension. It is 
difficult to design bits to induce di- 
rectly tensile (cleavage) fractures in 
rock without inducing tensile stresses in 
the tool material. Consequently, the 
loading arrangement usually configured is 
such that only compressive forces are 
applied to the bits and, by implication, 
compression forces (indentation loads) 
are applied to the rock. 

The weakest rock types are mined today 
not by drilling and blasting but in a 
cutting operation, machining the seam or 
ore directly from the face. Coal mining 
serves as an example. The advantage of 
this approach is that it permits the cy- 
clic drill-and-blast process to be re- 
placed by a continuous operation. The 
cutting machine becomes one unit in a 
system that cuts and loads the ore at the 
face and transports it from the mine. 
These cutting machines usually employ 



bits that break the rock in a cleavage 
process. Present technology also permits 
continuous excavation for tunnel drivage 
of medium and even strong rocks. The ma- 
chines in this case employ rolling cut- 
ters that break the rock by indentation. 
Excavation of these stronger rocks is 
feasible only where limited quantities of 
the rock are to be removed. Thus, at 
present , technology is not available for 
continuous excavation of these rock types 
for mining ore bodies , although there is 
considerable incentive in terms of in- 
creased productivity to achieve this 
goal. 

The strength of the bit material, the 
tungsten carbide inserts, controls the 
upper limit on the power that is trans- 
mitted to the bits. This is true whether 
the loading method is cleavage or inden- 
tation. It will be shown theoretically 
and demonstrated in practice (fig. 3) 
that when the power at the bit exceeds a 
critical level, this results in rapid 
thermal deterioration of the bit insert, 
which quickly leads to bit failure. The 
specific power, i.e., the power required 
to excavate unit area of the rockface, is 
given by 

P c = E c x r 



where P s = specific power, kW/m 2 , 



E s = specific energy, kJ/m 3 , 



and r = rate of excavation (face 
advance), m/s. 




Bit damaged by thermal overload 



FIGURE 3. - Bit failure due to thermal deterioration. 



Specific energy is defined as the ener- 
gy required to remove a unit volume of 
rock from the face. Thus, this term is a 
measure of the efficiency of the rock 
breaking process. The specific energy of 
a given breaking process depends only on 
that process and the rock type. Further- 
more, it has been noted that specific 
energy is related in an inverse power 
manner to the mean size of the rock frag- 
ments produced (fig. 4). 

The following statements can now be 
made. First, from the expression above, 
it is apparent that since specific energy 
is a constant for a given breaking pro- 
cess, the rate of rock excavation is 



directly proportional to the power trans- 
mitted to the rock. If this power is to 
be transmitted only through cutting bits, 
then the fundamental limitation on the 
rate of rock excavation is the strength 
of the bit materials. Two approaches to 
overcome this limitation seem feasible. 
Either the strength of the bit material 
could be increased, or some of the power 
required to break the rock could be sup- 
plied by means other than through a 
cutting bit. The former approach is a 
problem for the materials scientist. An 
example of the latter approach is the use 
of explosives, placed in drilled holes, 
to cause rock fracture. 



Second, from figure 4 it can be seen 
that explosives are an efficient means 
of rock excavation. Mechanical breaking 
methods — percussive and drilling, roller 
bit boring, and drag bit cutting — are 
moderately efficient processes. The in- 
efficient techniques are the more exot- 
ic breaking methods: electron beam guns, 
lasers, flame jet piercing, and water jet 
erosion. These inefficient processes re- 
quire several orders of magnitude more 
energy than explosives to break the rock. 
This result was, probably, the main find- 
ing to come from the flurry of research 
activity, conducted in the late 1960's 
and 1970' s, into rock breaking processes 
(5-7). This work was conducted to deter- 
mine the feasibility of applying these 
so-called exotic techniques for the pur- 
pose of rock excavation. In most cases 
it was found that, while technically 
feasible, the processes did not warrant 
further development because of the very 
large additional energy expenditures 
required. 

Third, although the logarithmic rela- 
tionship between particle size and energy 
has been known since the last century and 
various workers have ascribed empirical 



a. 

CO 



I0 5 



I0 4 



f 103 



102 



10' 



Diamond 
cutting 

Percussive- 
drilling 



-Jet piercing 
^Erosion drilling 



-Drag-bit cutting 
"Roller bit boring 

Impact- 
driven 
^wedge 

Explosives" 



I0 l 
0.01 



_L 



0.1 



100 



1,000 



I 10 
SIZE, mm 

FIGURE 4. - Specific energy as a function of 
particle size for variousrock-breaking processes. 

relationships to describe this behavior 
(8), a fundamental understanding of the 
rock fracture mechanisms that caused this 
phenomenon has yet to be derived. An im- 
proved understanding of these processes 
may well enable the development of more 
efficient rock breaking techniques. 



THE DEVELOPMENT OF WATER- JET-ASSIST TECHNOLOGY 



The development of a hybrid cutting 
system, using moderate-pressure water 
jets in combination with a mechanical 
tool, was inspired originally by a need 
to overcome a limitation alluded to 
above, i.e. , thermal deterioration of the 
tool when power needed to achieve reason- 
able penetration rates was applied ( 9_) . 
In this application, drag bits were being 
used to cut in strong, abrasive rocks. 
It was discovered that suitably directed 
jets, at pressures less than 10,000 psi, 
would reduce the forces acting on the bit 



substantially. In the laboratory, cut- 
ting in norite, the bit cutting force 
was reduced by a factor of about three 
times (fig. 5^4). Underground, cutting 
in quartzite, this force component was 
reduced about five times. Other results 
of equal significance from this test 
suite were the findings that the bit 
normal force was reduced even more dra- 
matically than the bit cutting force 
(fig. 5B). Also, the bit temperatures 
were reduced substantially when the jets 
were employed. 



200 
160 



LU 
O 

o 



-i — i — i — i — i — i — i — i i r 

Approx line of max available force 



KEY 

• With water jets 




.4, Peak cutting force 
j I i I i_ 



* No water jet — 

I L 



200 


1 


II 1* ' 


1 ' 1 


i 


1 ' 


_ 


160 
120 




• 


• 


• 
• 


• 
• 


- 


80 












- 


40 


1 


Peak penetrating 
1 i 1 i 


force 
1 i 1 


i 


I 


— 



2 4 6 8 10 12 

DEPTH OF CUT, mm 

FIGURE 5. - Bit cutting {A) and normal {B) 
forces while cutting in norite with and without 
water-jet assist. 

Other workers (10-12) also using drag 
bits assisted by moderate-pressure water 
jets have cut a variety of medium and 
strong rocks. These researchers have 
found similar substantial bit force re- 
ductions when the jets are employed. 
Figure 6 gives the results of cutting in 
Dakota sandstone with and without water 
jets. It appears that in all cases at 
least a 40- to 50-pct reduction in the 
bit cutting force was realized. Also, 
the bit normal force was reduced more 
than the bit cutting force. A further 
substantial benefit from the use of these 
jets was revealed by Tomlin (12) , who 
conducted his experiments on a roadheader 
mining machine at an underground test 



O 

LlI 
O 

a: 
o 







i i r 

KEY 

* Without water jet 

• With water jet, 
10,000 psi pressure A 



Drag 

_] 



..u 
.5 


A 


1 

A 


1 

A 


1 

A _ 


.0 








• 


.5 








— 






• 
1 


• 
1 


Normal 

1 



0.2 0.4 0.6 0.8 

BIT PENETRATION, in 

FIGURE 6. - Bit forces with and without jet 
assistance, cutting in Dakota sandstone. 

site. Using this machine it was discov- 
ered that the water jets have a substan- 
tial health and safety benefit in that 
they cause the dust make at the cutter- 
head to be reduced significantly. The 
water jets also decrease, and perhaps 
even eliminate, the incidence of fric- 
tional sparking. 



IMPLICATIONS OF THE RESEARCH FINDINGS 



The ability to reduce bit forces sub- 
stantially overcomes the fundamental re- 
quirement of limiting power to the bit to 
prevent its deterioration. Thus drag 
bits now find application in rock types 
where previously the rock was considered 
too strong. 

Many machines that employ drag bits as 
cutting tools are limited in terms of the 
maximum torque that they can exert at 



the cutterhead. Furthermore, it is found 
that the stresses imposed on the gear- 
trains to the head are proportional to 
the torque to the fifth power, that is, 



a = T5 



where 



and 



a = stress, 
T = torque. 



Thus, if the cutterhead torque is dou- 
bled, the stresses in the geartrain are 
increased by a factor of about 32 times. 
Conversely, of course, if the torque is 
halved, the stresses are reduced in like 
manner. The ability to reduce the bit 
cutting forces consistently by a factor 
of at least 2 implies that drum torques 
could be halved while the rate of mining 
is maintained. In some applications this 
has an obvious potential for improving 
machine reliability. 

Bit failure often is induced by 
high normal forces acting on the car- 
bide insert. The ability to reduce this 
force component even more dramatically 
than the bit cutting force component 
implies that the bit failure rate might 
be decreased substantially by the use of 
moderate-pressure water jets. This would 
have two consequences: (1) The direct 
cost of bits would be decreased, and (2) 
the time lost changing bits, which can 
be a significant portion of the overall 
downtime (13) , would be reduced. 

The ability to reduce bit temperatures 
while cutting also would substantially 
reduce the rate of bit wear and thus the 
bit failure rate. Significant reductions 
in bit temperatures have been reported 
when water jets are used despite the fact 
that the depth of cut has been increased 
to a value where the bit cutting force is 
the same with jets as without. Other 
evidence for substantial reductions in 
bit temperatures comes from field trials 
with a roadheader where workers report 
that the bits are cool enough to touch 
when they exit the cut. Why the bits are 
cooled to the extent reported is not 
clear. Calculations predict that the 
convective cooling effect of a jet would 
not significantly increase the heat 
transfer from the bit-rock interface. 
The model used in these calculations as- 
sumes that heat is generated by friction 
beneath the bit wearflat. It is assumed 
that the water contacts the leading face 
of the bit but does not penetrate beneath 



the bit wearflat. This second assumption 
is based on simple calculations that show 
that the pressure beneath the wearflat 
generally is greater, by an order of mag- 
nitude or more, than the pressure of 
these moderate-pressure jets. If the 
second assumption is in error and water 
does penetrate beneath the wearflat, then 
this could result in significant bit tem- 
perature reductions. Recent work by 
Friedman ( 14 ) on the fracture pattern 
left in a rock groove cut by a drag bit 
indicates a possible mechanism for trap- 
ping water in preexisting flaws ahead of 
the bit. This water then would be con- 
strained beneath the wearflat with the 
forward passage of the bit. 

The potential for a dramatic reduction 
in the incidence of frictional sparking, 
a major concern in gassy mines, is almost 
certainly a consequence of the reduced 
heat loads on the bit. The predominant 
cause of frictional ignitions and explo- 
sions in coal mines is sparking between 
the bits and the rock (15) . Furthermore, 
the incidence of frictional ignitions 
continues to increase, reflecting the in- 
crease in mechanization (15). Ignitions 
occur in appropriate methane-air mixture 
when the energy level of a heat source 
is above a critical minimum level. The 
apparent substantial reduction in the 
heat levels generated within the bits 
when moderate-pressure water jets are 
used to assist the cutting operation 
would explain the observed reduction in 
ignitions. 

Another safety and health hazard, which 
apparently is reduced substantially by 
the use of a water-jet-assist system, is 
mine dust. It is generally accepted that 
the primary source of dust in an under- 
ground mine where excavation is carried 
out using a cutting machine is the bit- 
rock interface. Past attention to con- 
trolling this hazard has concentrated on 
directing the airflow to move the dust 
particles away from the workers and in- 
stalling scrubbers on machines to remove 



10 



dust particles from the airstream. 
Despite considerable advances in these 
areas , directed mainly by the Bureau 
of Mines over the last 15 years, there 
probably is not one longwall coal face 
in the United States today that is in 
regular compliance with the very strict 
dust standards. This water-jet-assist 
approach appears to offer the potential 



for preventing small particles from be- 
coming entrained in the airstream; that 
is , it may inhibit the generation of dust 
particles. This may herald a major ad- 
vance in dust control since it is evi- 
dent that inhibition of dust formation is 
a superior control strategy to dust 
suppression. 



REMAINING TECHNOLOGICAL PROBLEMS 



Despite the considerable promise of 
water-jet-assist technology, its applica- 
tion to mining machines has been slow. 
Two reasons for this can be found. 
First, current understanding of the mech- 
anism by which the jets assist the break- 
ing process is poor. It might be argued 
that such understanding is of academic 
interest only; after all, stone age man 
fashioned arrowheads without a grasp of 
the theories of fracture mechanics. How- 
ever, an examination of the results of 
various workers (figs. 5-6) reveals that 
bit force reductions of factors of less 
than 2 to as much as 5 have been report- 
ed. Parameters that affect these force 
reductions include jet pressure, jet flow 
rate, jet position (with respect to the 
bit-rock interface) , the standoff dis- 
tance between the water jet nozzle and 
the rock, bit geometry, bit velocity, and 
rock type. Until recently the relative 
importance of these parameters in influ- 
encing bit force reductions has been un- 
clear. Evidently, a machine designer 
needs guidance in this area in order to 
incorporate a water jet system onto a 
mining machine. 



Second, although the jet pressures at 
which these systems operate are not high, 
some development of hardware to enable 
the jets to be channeled to the bits is 
required. This hardware includes a reli- 
able swivel and a phasing system to en- 
sure that only those bits actually in 
contact with the rock are assisted. 

The following section addresses the 
first of these problem areas. The devel- 
opment of an hypothesis to describe a 
physical process follows one of two ap- 
proaches . In the one approach a mathe- 
matical model of the process is proposed 
and experiments are conducted subsequent- 
ly to verify the model. The alternate 
method is to conduct experiments first 
and to use the empirical data that are 
generated to derive the mathematical mod- 
el. The latter approach has been adopted 
in this investigation. The work is not 
yet complete. The present study de- 
scribes some key experiments which add to 
the data bank of empirical knowledge in 
this work area and which suggest physical 
processes by which the jets act to assist 
rock breakage. 



EXPERIMENTAL PROCEDURES AND RESULTS 



Experiments were conducted in the la- 
boratory cutting the rock in a linear 
planing machine. These tests were car- 
ried out to determine the relative impor- 
tance of the parameters listed above on 



the reduction of bit forces using water 
jets. To limit the experimental program 
to a manageable size, the parameters in- 
vestigated were limited. Parameters not 
studied in this test series were bit 



11 



geometry, rock type, depth of cut, and 
nozzle geometry. A V-faced chisel, or 
radial, pick was employed. All cutting 
experiments were conducted in Indiana 
Limestone. The properties of this rock 
are described by Krech (16). The depth 
of cut taken was 15 mm. The nozzle em- 
ployed for the water jet used a 13° 
included angle convergent section and a 
parallel section at the exit. This de- 
sign performed well in tests conducted 
by Leach and Walker ( 17 ) . The parameters 
studied follow: 

1. Jet position. Three jet configura- 
tions were examined (fig. 7): 

A. Two jets parallel to the lead- 
ing face of the pick, about 1 mm ahead 
of this face. 

B. A single jet parallel to the 
leading face of the pick, aligned in 
the center of the tungsten carbide in- 
sert, again about 1 mm ahead of the 
face. 

C. A single jet directed behind 
the pick in the path of the pick. 

2. Standoff distance. Using the opti- 
mum jet position (above), three standoff 
distances were examined: 25, 50, and 
75 mm. 

3. Jet pressure . A range of jet pres- 
sures up to a maximum of 10,000 psi was 
tested. 

4. Jet flow rate . Three different 
nozzle diameters were tested in this test 
suite at jet pressures of 0.6, 0.8, and 
1.0 mm. The flow rates corresponding to 
these nozzles at various pressures are 
given below. 

5. Pick velocity . Two pick velocities 
were examined: 0.06 and 0.25 m/s. 




Nozzle 



Side view 



•Nozzle 



Front view 



Nozzle 
.holder 

Water inlet 




Adjustable standoff 1 
25 mm, 50 mm, 75 mm 



Side view 

Nozzle 




Front view 



Tungsten 
carbide tip 



tmsm 




Nozzle 



mm> 



50 mm 

FIGURE 7. - Positioning of the jets with re- 
spect to the bit. A, Two jets ahead of the bit; 
B, single jet ahead of the bit; C, single jet be- 
hind bit. 



JET POSITION 



It was found that the optimal jet ar- 
rangement for this bit cutting in this 
rock type was a single jet directed 1 mm 
ahead of the leading face of the bit. 
The finding that the optimal jet position 
is ahead of the bit contradicts results 
reported by Ropchan (10) in which it is 
claimed that the greatest force reduc- 
tions were achieved when a single jet was 
directed behind the bit. The major dif- 
ference between the present investigation 
and the program conducted by Ropchan is 



the bit geometry. In the earlier study a 
point attack bit was used for the cutting 
experiments that employed water-jet as- 
sist. Only modest bit force reductions 
were reported when the water jet was di- 
rected ahead of the bit. It may be that 
the geometry of the point attack bit is 
not capable of exploiting fully the ad- 
vantages of assistance with water jets. 

It should be pointed out that previous 
work has shown that close proximity of 
the jet to the leading face of the bit is 



12 



crucial to obtaining significant bit 
force reductions. If, during the cut, 
the jet strikes the rock 10 mm or even 5 
mm ahead of the bit, its effectiveness in 
reducing the bit forces is decreased dra- 
matically. On the other hand, it is im- 
portant to insure that the jets do not 



strike the tungsten carbide insert be- 
cause they cause rapid erosion of this 
insert. Accurate positioning of the jet 
with respect to the bit is a crucial fac- 
tor in obtaining maximum benefit from the 
water jets. 



STANDOFF DISTANCE 



The standoff distance was found not to 
affect the bit force reductions provided 
that this distance was less than 100 
times the jet nozzle diameter. However, 
at distances greater than this, the re- 
duction in these forces fell rapidly. 
This finding should be qualified since, 
almost certainly, the result depends on 



the nozzle geometry and on the inlet con- 
ditions of the nozzle. The usefulness of 
this result probably is that it provides 
a rule of thumb for calculating an ac- 
ceptable standoff distance. One justifi- 
cation for using this result is that it 
accords with other, more precise, labora- 
tory measurements (17). 



JET ENERGY 



During the cutting operation the reduc- 
tions in the bit forces were found to be 
affected both by the pressure at which 
the jet was operating and by the flow 
rate of the jet. However, careful analy- 
sis of the experimental data revealed 
that it was the combination of these two 
parameters, the jet power, that con- 
trolled these reductions. The results 
indicated that the magnitude of the re- 
ductions in bit forces is some function 
of the jet power normalized with respect 
to the bit velocity. This parameter has 
units of kilojoules per meter, or jet en- 
ergy per unit length of cut. A typical 
curve for a slow cutting speed is given 



in figure 8. This result, in general, 
accords with intuition. It would be ex- 
pected that, over a certain length of the 
cut, the bit force reductions would in- 
crease as the energy of the jet in- 
creased. In addition, it may be antici- 
pated that these force reductions would 
reach a limit beyond which an increase in 
the jet energy would produce little or no 
effect. The results obtained indicate 
that these force reductions attain a max- 
imum. When the normalized jet power is 
increased beyond this maximum, the bit 
force reductions again start to decrease. 
This behavior is not well understood. 



BIT VELOCITY 



The tests described above all were con- 
ducted at a pick velocity of 0.06 m/s. 
The maximum possible bit velocity in this 
test suite was constrained by the experi- 
mental apparatus to 0.25 m/s. Although 
this represents an increase in speed of a 
factor of 4, it still is about four times 
less than typical bit speeds on mining 
machines. Nevertheless, a comparison of 
tests conducted at these high and low 
laboratory bit velocities is of interest 
since it reveals the trend in the influ- 
ence of jet parameters on bit force 
reductions. 




1 r~ 

KEY 

- Slow speed 

— Fast speed 



_L 



50 100 150 200 250 300 

JET ENERGY PER UNIT LENGTH, kJ/m 

FIGURE 8. - Measured bit force as a function 
of the normalized jet power. 



13 



Results from experiments carried out at 
the higher bit speed of 0.25 m/s are also 
plotted as a function of normalized jet 
power in figure 8. Inspection of this 
curve shows that the trend observed pre- 
viously for the lower speed tests, i.e., 
a rapid decrease in the bit forces with 
increasing jet energy up to some maximum 
jet energy, is reproduced at the higher 
bit speed. Moreover, visual inspection 
of these curves shows that their sta- 
tionary points correspond to bit force 
reductions of about 50 pet at the low 
bit speed and about 45 pet at the high 
bit speed. The corresponding jet ener- 
gies per unit length of cut are about 30 
and 120 kj/m, respectively. It might be 
concluded that provided that an appropri- 
ate energy is supplied to the jet, the 
bit force reductions are not affected 
significantly by changes in bit velocity. 
Furthermore, the jet energy appears to 
increase linearly with bit velocity. 

However, closer inspection of these 
curves reveals that the maximum force 



reductions are not necessarily the opti- 
mum operating points. Because, in most 
cases, the rate of change of increase in 
force reductions slows at some fairly 
well defined point in the curve and the 
difference in force reductions between 
this point and the maximum is small, this 
point of change may be the most energy 
efficient point at which to operate the 
system. At present the analysis has 
progressed only to the point of identi- 
fying these optimum points by visual 
inspection. These are marked in fig- 
ure 8. It can be observed that in this 
figure the normalized jet power changes 
by a factor of 2 while the bit velocity, 
as noted, changes by a factor of 4. 
This result was repeated using two other 
nozzle sizes. 

Extrapolation of these admittedly lim- 
ited data to calculate the probable jet 
power requirement for bits cutting at 
velocities of 1 m/s indicates that the 
jet power to an individual bit may be 
about 15 kW. 



INVESTIGATION OF BREAKAGE MECHANISMS 



An additional suite of experiments was 
performed. These experiments were de- 
signed specifically to throw light on the 
mechanisms of rock breakage when water 
jets at moderate pressures are employed 
to assist rock cutting with drag bits. 

Three mechanisms by which the jets 
might act to reduce the bit forces were 
proposed: 



1. Chemical attack of the rock by the 
water, i.e., stress corrosion cracking. 

2. Initiation of a crack by the bit at 
low bit-force levels, and subsequent 
propagation of the crack by the water jet 
to form a chip ahead of the bit. 

3. Effective clearance of the rock 
particles from the region adjacent to the 
bit. 



STRESS CORROSION CRACKING 



It is known that stress corrosion 
cracking (SCC) can produce significant 
reduction in the fracture strength of 
rock (18-20) . However, the effectiveness 
of this approach in reducing fracture 
toughness decreases as the velocity of 
the crack front increases. This study 
was conducted to determine whether this 
mechanism could be effective when frac- 
tures are produced in a dynamic manner. 

A series of high-speed films of the 
cutting operation was made using a film 
speed of 1,000 frames per second. Films 
were made both with and without the 
use of water jets to assist the cutting 



process. The approximate speed of crack 
propagation for the large rock chip that 
forms ahead of the bit was determined 
from careful viewing of these films. 
These chips typically were 80 mm in 
length and they were formed within one 
or, at most, two frames on the film. 
This implies a crack propagation velocity 
of about 80 m/s. Stress corrosion crack- 
ing is known to be a rate-dependent phe- 
nomenon, i.e. , a unique relationship ex- 
ists between the rate of change of crack 
length with respect to time and the 
stress intensity factor at the crack 
tip (21). Furthermore, there exists a 



14 



limiting crack velocity beyond which 
stress corrosion plays no part in the 
fracture process. For most brittle mate- 
rials, including rock, this limiting ve- 
locity appears to be of the order of 10~ 4 
m/s to 10" 1 m/s (21). Thus, the fracture 
velocity observed in these cutting tests 



is at least two orders of magnitude high- 
er than the maximum velocity at which SCC 
could be invoked as a mechanism for re- 
ducing the fracture strength and thereby 
reducing the bit forces. Therefore, this 
hypothesis was rejected. 



DRIVAGE OF CRACKS BY WATER PRESSURE 



In addition to the large chip that is 
formed ahead of the bit in its forward 
passage through the rock, a large number 
of small rock chips also are created. A 
two-dimensional representation of this 
breakage process is shown in figure 9. 
Immediately after the formation of a 
large chip the depth of the cut seen by 
the bit is zero. As the bit continues 
its forward passage, the depth of cut in- 
creases and small rock fragments are 
broken ahead of the bit; these fragments 
are pushed ahead of the bit. Although 
details of the fracture process that pro- 
duces these fragments still are not well 
understood, almost certainly a tensile 
crack is initiated in the rock adjacent 
to the corner of the bit between the 
leading bit face and the bottom surface 
of the bit for the case when the drag bit 
is sharp. In other words, it is a 
cleavage-type crack. On the other hand, 
when the drag bit is blunt, i.e., a wear- 
flat exists because of a zero clearance 

Rock surface left 
by previous chip - 




^$SSS35^SSSS5^ 



Rock crushed ahead of bit, 
applying indentation load to the rock 




Compacted crushed 
Scale, mm material beneath 

FIGURE 9. - Chipping process ahead of 
sharp drag bit. 



angle between the bottom surface of the 
bit and the rock, then this tensile crack 
is initiated by indentation. In these 
processes it is known that crack initia- 
tion occurs with relatively low forces 
applied to the bit. A considerable in- 
crease in the bit force is required to 
cause this fledgling crack to propagate 
to form a rock chip. 

Thus, a plausible explanation for the 
influence of water jets in reducing bit 
forces is that the jet enters the crack 
initiated by and ahead of the bit, and 
the jet energy causes this crack to prop- 
agate to form a rock chip. Evidence sup- 
porting the mechanism of rock breakage 
was provided in a series of indentation 
experiments reported by Hood (22). In 
these tests rock specimens were loaded in 
a quasi-static manner in a series of 
punch tests with a blunt drag bit serving 
as the punch. It was found that when wa- 
ter jets were directed, at moderate pres- 
sure, onto the rock surface immediately 
adjacent to the bit, the force required 
to cause the rock to fail and a chip to 
form was reduced significantly. The ex- 
planation given for this reduced bit 
force was that full development of the 
fracture that produces the large rock 
chip ahead of the bit occurs only after 
the rock beneath the bit is crushed in an 
energy-consuming and inefficient process. 
By driving the crack with the water jet 
less crushing of the rock by the bit took 
place , and thus mechanical energy that 
normally would have been supplied by the 
bit was saved. 

In the present test series the drag bit 
used was sharp and the purpose of this 
investigation was to determine whether a 
similar mechanism for crack drivage by 
water pressure was employed. It was ar- 
gued that smaller quantities of finely 
crushed rock particles would be formed if 



15 



jet pressure was responsible for driv- 
ing the cracks that formed the major 
rock chips. Experiments were conducted 
in which all of the fragments produced 
during the cutting operation were col- 
lected. Size analyses of these particles 
revealed no discernible difference in 
the distribution of particle size with 
and without water jet assistance of the 
cutting process. A preliminary conclu- 
sion was drawn that the efficiency of 
the cutting process is not increased by 
crack drivage with water pressure when 



moderate-pressure jets are used to assist 
sharp drag bits. This conclusion must be 
regarded as tentative since the experi- 
mental technique used, analysis of the 
particle size, is not definitive. For 
example, although the quantity of fines 
was found to be the same in cuts made 
both with and without the use of water 
jets, the fines in the water-jet-assisted 
cuts may have been formed by erosion of 
the chip edges by the jets, while the 
fines in the dry cuts were produced by 
crushing. 



CHIP CLEARANCE BY JETS 



When water jets are not used during the 
cutting operation, the rock chips formed 
prior to the formation of a large chip 
are pushed ahead of the leading bit face. 
Therefore, stresses applied to the intact 
rock by the bit are transmitted through a 
region of crushed material. This materi- 
al acts as a cushion, and the applied 
stresses are distributed more evenly and 
over a larger area than would be the case 
if no crushed material was present. This 
results in higher bit forces causing rock 
breakage. 

Also, it is known that the strength of 
rock is increased dramatically by the ap- 
plication of confining pressure even when 
the confining pressure is small '(23) . 
The weight of the rock fragments that are 
pushed ahead of the bit in its forward 
progress through the cut is so small as 
to be negligible in terms of the confine- 
ment that it could provide. However, 
it is possible or even likely that this 
crushed material could be wedged between 
the front face of the bit and the in- 
clined surface of the intact rock face 
(fig. 9). In this case appreciable con- 
fining pressure might be applied to the 
underlying rock, and thus the energy re- 
quired to propagate a crack to form a 
rock chip would be increased. The like- 
lihood of wedged rock fragments confining 
the rock in this manner would seem to be 
higher for bits with either zero or nega- 
tive rake angles. Cutting experiments 
(24) have shown that bit forces indeed 
are increased as the bit rake angle 
changes from positive through zero to 
negative. Since the fracture process is 



tensile in all cases, this mechanism 
of confinement of rock particles may pro- 
vide the explanation for these force 
differences. 

Therefore, the hypothesis developed to 
explain the action of the jets in reduc- 
ing bit forces was that they removed 
rock debris as it was formed ahead of the 
advancing bit . This would permit higher 
stresses to be applied to the rock by re- 
moving the cushion of crushed material. 
Also, any enhancement in rock strength as 
a result of confining stress applied 
through this debris would be removed. 

Experiments were conducted to investi- 
gate the effect of confinement of rock 
particles ahead of an advancing drag bit. 
A plate fixture was mounted in front of 
the bit 25 mm above the rock surface 
(fig. 10). This plate prevented upward 
movement by the rock chips once the 
height of the debris in front of the bit 
exceeded 25 mm. In other words , this 
fixture caused the effect of confinement 
of the rock chips to be exaggerated. 
Tests were conducted both with and with- 
out the use of water jets assisting the 
cutting process. 

Representative samples of the cutting 
force-time traces that were recorded both 
with and without the fixture and with and 
without the use of water jets are given 
in figures 11 and 12. Figure 11 A shows 
the typical sawtooth trace obtained for a 
dry cut without the fixture. The cutting 
force increases from zero, or some low 
value, in an oscillatory but linear man- 
ner, up to a maximum. Beyond this maxi- 
mum value the force decreases rapidly to 



16 



25-mm-wide 
metal fixture 



15-mm 
cut depth 




^^^^^^m^^^^^ 



Crushed zone 



FIGURE 10. - Plate fixture ahead of drag bit. 




12 3 4 

TIME, s 

FIGURE 11. - Dry cuts without (.4) and with 
(B) plate fixture. 

a value close to zero, and the cycle is 
repeated. This characteristic signature 
is explained in terms of the observed 
chip formation process in front of the 
advancing bit (fig. 9). Immediately af- 
ter a large chip is formed, the instan- 
taneous depth of the cut is low or zero, 
and thus the bit cutting force is low or 
zero. As the bit advances it encounters 
a ramp formed by the bottom surface of 
the previous chip. Thus, the depth of 
cut that the bit sees increases, in an 
approximately linear fashion, from a val- 
ue close to zero to the depth of cut pre- 
determined in the experiment. The forma- 
tion of small rock chips as this ramp is 
excavated together with crushing of those 



UJ 

o 
rr 
o 




TIME, s 
FIGURE 12. - Water-jet-assisted cuts without 
{A) and with (B) plate fixture. 

chips as they are pushed ahead of the bit 
accounts for the oscillatory nature of 
the trace. The force continues to in- 
crease because the depth of cut relative 
to the bit is increasing. At some point 
the pressure beneath the crushed material 
is sufficient to initiate and propagate 
another major chip. The instantaneous 
depth of cut ahead of the bit returns to 
a low value, and the cycle is repeated. 

When the metal fixture was used for dry 
cuts, this same signature was repeated 
but it became much more exaggerated (fig. 
116). The area beneath each sawtooth 
oscillation, representing the mechanical 
energy required to form one large rock 
chip, is much increased. 



17 



The equivalent traces without and with 
the metal fixture mounted ahead of the 
bit but with water-jet assistance are 
given in figures 12A and 125, respective- 
ly. A notable feature of these plots is 
that although saw-tooth oscillations 
still can be distinguished, in general, 
they are of short duration so that the 
area under the curves , and thus the ex- 
penditure of mechanical energy, is small. 
Little difference can be observed by 
visual inspection of these plots. This 
small difference is reflected in the rel- 
atively small difference in the measured 
cutting forces when jets were used. 
These forces for this set of experiments 
are given in table 1. The mechanical 
energy supplied to the bit in these ex- 
periments is given in table 2. 

TABLE 1. - Bit forces with and without 
metal plate, kilonewtons 



TABLE 2. - Mechanical energy supplied 
to the bit, kilo joules 



With water jets: 

Mean 

Mean peak 

Without water jets: 

Mean 

Mean peak 



With 


Without 


plate 


plate 


2.95 


2.24 


7.99 


6.82 


6.63 


3.94 


12.40 


8.45 



With water jets..., 
Without water jets, 



With 
plate 



3.47 
7.76 



Without 
plate 



2.62 
4.62 



High-speed films made of the cutting 
operation when water-jet assistance was 
employed showed that rock particles 
were removed from the region ahead of 
the bit immediately after they were 
formed. This contrasts with evidence 
from similar films made of the cutting 
operation when water jets were not used, 
which indicated that rock chips were 
carried ahead of the bit for the com- 
plete length of the cut. Further evi- 
dence as to the effectiveness of this 
flushing operation using jets was pro- 
vided by examination of the groove after 
the bit had passed. With dry cuts this 
groove bottom was packed with finely 
crushed material which obviously had been 
forced and/or trapped beneath the bit 
wear-flat. With water-jet-assisted cuts 
this groove bottom contained no crushed 
material. 



DISCUSSIONS OF FINDINGS AND CONCLUSIONS 



The experimental evidence supports the 
hypothesis that the dominant mechanism by 
which moderate-pressure water jets act 
to assist the rock cutting process using 
sharp drag bits is by effective chip 
clearance from the region ahead of the 
bit. Removal of these rock particles 
causes a reduction in the measured bit 
forces because the stresses transmitted 
to the rock by the bit are increased 
for a given bit load. In addition, the 
confining stress that may be applied by 
these rock particles to the intact rock 
when cutting dry is removed. This also 
would cause a reduction in the bit forces 
necessary to excavate the rock. 

It was noted above that in order to 
reduce bit forces substantially it is 
important that the jet be positioned 
within 1 or 2 mm of the leading face of 
the bit. The necessity for this accurate 



positioning of the jet was provided by 
the high-speed films of the cutting oper- 
ation. When jets are not used, platelike 
particles form and are pushed ahead of 
the bit. If the jet is directed too far 
ahead of the leading bit face, it strikes 
the center of one of these particles and 
the jet energy, which is insufficient 
to damage the rock, is dissipated harm- 
lessly. On the other hand, when the jet 
is directed immediately adjacent to the 
leading bit face, it penetrates beneath 
the edge of these particles and the ener- 
gy dissipated as it strikes the intact 
rock ahead of the bit lifts and removes 
all debris in this region. 

The experimental finding that jet ener- 
gy does not have to increase linearly 
with bit velocity is consistent with this 
hypothesis for jet assistance, i.e., that 
the dominant influence of the jets is 



18 



removal of broken material from ahead of 
the bit. 

Another possible mechanism by which the 
jets may assist the cutting process is 
crack drivage by jet pressure. Apparent- 
ly this does occur with blunt bits and 
may or may not take place with sharp 
bits; the experimental results are un- 
clear. The third possibility for reduc- 
ing bit force as examined in this study 
was stress corrosion cracking. Apparent- 
ly this does not influence the breakage 
process . 

A substantial reduction in bit tempera- 
tures by the use of water jets is not 
predicted by theory if the assumption is 
made that the dominant mode of heat 
transfer is convective and that cooling 
takes place from the leading bit face. 
However, laboratory and field tests have 
shown that bit temperatures are reduced 
substantially when jets are used. What- 
ever the flaw in the mathematical model, 
the fact that heat loads to the bit dur- 
ing the cutting operation are signifi- 
cantly decreased when jets are used al- 
most certainly accounts for the observed 
dramatic reduction in frictional spark- 
ing. Furthermore, this finding, taken 
together with reduced bit normal force 
that is observed even when the depth of 
cut is increased so that the bit cutting 
force is not decreased, probably accounts 
for the reported reductions in bit wear 
and breakage. 

Dust might be defined as those fine 
particles that become entrained in the 
air. On a coal face it is known that 
dust accounts for only a very small frac- 
tion of the fine particles that are gen- 
erated during the cutting operation. Re- 
sults from these experiments indicate 
that the quantity of fines produced is 
not different when water jets are used. 
However, measurements show that dust 
quantities at the face are reduced sub- 
stantially when jets are used to assist 
the cutting process (12) . Thus, it must 
be assumed that the effect of the jets is 
to wet the fine particles as they are 
produced at the face before they become 
entrained in the ventilation airstream. 
This assumption is consistent with the 
model proposed in this paper to describe 
the dominant effect of the jets on the 



rock fracture process, i.e., that the 
jets act to flush chips and rock debris 
from the region ahead of the bit . For 
this flushing operation to be most effec- 
tive, the jet is aimed directly at the 
small region where the rock particles, 
both small and large, are initiated. 
Thus, before these particles are removed 
from the face they have been wetted by 
the jet. These wetted particles are much 
less likely to become entrained in the 
airstream. 

Another factor with a high potential 
for achieving even further reductions in 
the dust make is the possibility of re- 
designing the cutting machine to take 
deeper cuts at lower bit velocities. It 
has been known for decades that fewer 
fines are produced by deep, widely spaced 
cuts (25). It is known also that the en- 
trainment process has much to do with 
bit, or drum, velocity. The problem with 
putting this knowledge into practice 
on mining machinery has been that this 
cutting method places high unbalanced 
loads on the drum and that high drum 
torque is needed. Water-jet assistance 
offers the potential for overcoming these 
difficulties. 

The other research results are summa- 
rized as follows. Parameters influencing 
the effectiveness of water jets in reduc- 
ing bit forces were examined. It was 
found that the optimal jet position for 
assisting a sharp chisel bit was immedi- 
ately ahead of the leading face of the 
bit. The jet should strike the rock no 
more than 1 or 2 mm ahead of this face. 
The standoff distance will depend on the 
nozzle geometry and the nozzle inlet con- 
ditions. It was found in these tests 
that the nozzle exit should be within 100 
nozzle diameters to be most effective. 
Although jet pressure and jet flow rate 
both affect the reduction in the bit 
forces, the jet power per unit length of 
cut was found to be the parameter that 
controlled the magnitude of these force 
reductions. The optimal value of this 
normalized jet power parameter was found 
to depend on bit velocity. An extrapola- 
tion from limited experimental data 
yielded an estimate for the jet power of 
15 kW per bit for bit velocities of about 
1 m/s. 



19 



REFERENCES 



1. National Research Council, Nation- 
al Materials Advisory Board. Comminution 
and Energy Consumption. Natl. Acad. 
Press, Publ. NMAB-36, 1981, 283 pp. 

2. Evans, I., and S. A. F. Murrell. 
Wedge Penetration Into Coal. Colliery 
Guardian, v. 39, No. 455, Jan. 1962, pp. 
11-16. 

3. Hood, M. Phenomena Related to the 
Failure of Strong Rock Adjacent to an 
Indenter. J. S. Afr. Inst. Min. and 
Metall., v. 78, No. 5, Dec. 1977, p. 113. 

4. Cook, N. G. W. , M. Hood, and F. 
Tsai. Observations of Crack Growth in 
Hard Rock Loaded by an Indenter. Int. J. 
Rock Mech. and Min. Sci., v. 21, No. 2, 
1984, pp. 97-107. 

5. Liessmann, Y. Das Mikrowellenver- 
fahren zur Zerkleinerung von Gestein- 
verschusanlage und Erzielten Ergebrisse 
(The Microwaves for Rock Breaking — 
Experimental Techniques and Results). 
Bergbautechnik, v. 16, No. 10, 1966, p. 
537. 

6. Carstens, J. P. Thermal Fracture 
of Rock — A Review of Experimental Re- 
sults. Paper in North American Rapid Ex- 
cavation and Tunneling Conference, ed. by 
K. S. Lane and L. A. Garfield (Proc. 
Conf., Chicago, IL, June 5-7, 1972). 
AIME, 1972, pp. 1363-1392. 

7. Summers, D. A., and D. J. Bush- 
nell. Preliminary Experimentation of the 
Design of the Water Jet Drilling Device. 
Paper E2 in Third International Symposium 
on Jet Cutting Technology (IIT Res. 
Inst., Chicago, IL, May 11-13, 1976). 
BHRA Fluid Engineering, Cranfield, Bed- 
ford, England, 1976, pp. E2-E21. 

8. Bond, F. C. Third Theory of Com- 
minution. Trans. AIME, v. 193, 1952, pp. 
484-494. 

9. Hood, M. Cutting Strong Rock With 
a Drag Bit Assisted by High Pressure Wa- 
ter Jets. J. S. Afr. Inst. Min. and 
Metall., v. 77, No. 4, Nov. 1976, pp. 79- 
90. 

10. Ropchan, D. , F. D. Wang, and J. 
Wolgamott. Application of Water Jet As- 
sisted Drag Bit and Pick Cutter for the 
Cutting of Coal Measure Rocks (U.S. Dep. 
Energy contract ET-77-A-01-9082, CO Sch. 



Mines). Apr. 1980, NTIS DOE/ET/ 1 2463-1 , 
120 pp. 

11. Dubugnon, 0. An Experimental 
Study of Water Assisted Drag Bit Cutting 
of Rocks. Paper in First U.S. Water Jet 
Symposium (Golden, CO, Apr. 7-9, 1981). 
CO Sch. Mines Press, Golden, CO, 1981, 
pp. II-4.1 to II-4.11. 

12. Tomlin, M. G. Field Trials With 
10,000 psi Prototype System. Paper in 
Seminar on Water Jet Assisted Roadheaders 
for Rock Excavation (Pittsburgh, PA, May 
26-27, 1982). U.S. Dep. Energy and U.K. 
National Coal Board, 1982, pp. Cl-Cll. 

13. Pimental, I. R. A., J. T. Urie, 
and W. J. Douglas. Evaluation of Long- 
wall Industrial Engineering Data (U.S. 
Dep. Energy contract ET-77-C-01-8915, 
Ketron Inc., Wayne, PA). 1981, 11 pp.; 
NTIS DOE/ET/ 12532-T2. 

14. Friedman, M. Analysis of Rock De- 
formation and Fracture Induced by Rock 
Cutting Tools Used in Coal Mining (U.S. 
Dep. Energy contract AC04-76DP00789, Tex- 
as A & M Univ., College Station, TX) . 
Sandia National Laboratory, Albuquerque, 
NM, Contractor Rep. SAND 83-7007, Mar. 
1983, 36 pp. 

15. Richmond, J. K. , G. C. Price, 
M. J. Sapko, and E. M. Kawenski. Histor- 
ical Summary of Coal Mine Explosions in 
the United States, 1959-81. BuMines 
IC 8909, 1983, 53 pp. 

16. Krech, W. W. , R. A. Henderson, and 
K. E. Hjelmstad. A Standard Rock Suite 
for Rapid Excavation Research. BuMines 
RI 7865, 1974, 29 pp. 

17. Leach, S. J., and G. I. Walker. 
The Application of High Speed Liquid Jets 
to Cutting; Some Aspects of Rock Cutting 
by High Speed Water Jets. Proc. R. Soc. 
London, Ser. A, v. 260, 1966, pp. 295- 
308. 

18. Hoagland, R. G. , G. T. Hahn, and 
A. R. Rosenfield. Influence of Micro- 
structure on Fracture Propagation in 
Rock. Rock Mech., v. 5, 1973, pp. 77- 
106. 

19. Westwood, A. R. C. Control and 
Application of Environment Sensitive 
Fracture Processes. J. Mater. Sci., v. 
9, 1974, pp. 1871-1895. 



20 



20. Schmidt, R. A. Fracture Mechanics 
of Oil Shale; Unconfined Fracture Tough- 
ness, Stress Corrosion Crackling, and 
Tension Test Results. Paper in Energy 
Resources and Excavation Technology 
(Proc. 18th Symp. on Rock Mechanics, Key- 
stone, CO, June 22-24, 1977). CO Sch. 
Mines Press, Golden, CO, 1977, pp. 2A2-1 
to 2A2-6. 

21. Barton, C. C. Variables in Frac- 
ture Energy and Toughness Testing of 
Rock. Paper in Issues in Rock Mechanics 
(Proc. 23d U.S. Symp. on Rock Mechanics, 
Univ. CA— Berkeley, Aug. 25-27, 1982). 
Society of Mining Engineers, AIME, 1982, 
pp. 449-462. 



22. Hood, M. A Study of Methods To 
Improve the Performance of Drag Bits Used 
To Cut Hard Rock. Ph.D. Thesis, Dep. 
Min. Eng. , Univ. Witwatersrand, Republic 
of South Africa, 1978, 150 pp.; available 
from M. Hood, Univ. CA, Berkeley, CA. 

23. Jaeger, J. C. , and N. G. W. Cook. 
Fundamentals of Rock Mechanics. Chapman 
and Hall, London, 3d ed. , 1979, 593 pp. 

24. Roxborough, F. F. Cutting Rocks 
With Picks. Min. Eng. (London), v. 132, 
No. 153, June 1973, pp. 445-455. 

25. Barker, J. S., C. D. Pomeroy, and 
D. Whittaker. The M.R.E. Large Pick 
Shearer Drum. Min. Eng. (London), v. 
125, No. 65, Feb. 1966, pp. 323-333. 



21 



ANALYSIS OF MECHANICAL TOOL FORCE REDUCTIONS 
WHEN USING WATER-JET-ASSISTED CUTTING 

By R. J. Evans, 1 H. J. Handewith, 2 and C. D. Taylor 3 



ABSTRACT 



Water-jet-assisted cutting is the syn- 
ergistic combination of a mechanical pick 
and a directed moderate-pressure water 
jet which greatly facilitates the cutting 
process. The water jet assists the me- 
chanical pick by lubricating the cutting 
process, by cleaning the crushed zone 
directly in front of the bit to reduce 
friction, by cooling the mechanical tool 
to improve bit life, and by exploiting 
cracks close to the bit to promote chip 
fragmentation. 

An In-Seam Tester, which is a 
hydraulically activated single pick 



instrumented to measure and record cut- 
ting forces, was designed and fabricated 
by the Bureau of Mines to obtain design 
data for a rotary drum cutting system 
using moderate-pressure jets (3,000 to 
10,000 psi) . Data are presented for 
cutting trials in a simulated coal block, 
underground in the Pittsburgh coal seam, 
and on test blocks of sandstone and 
limestone for water-jet-assisted and dry 
cutting. Data are presented which indi- 
cate that pick forces are significantly 
reduced when using water-jet-assisted 
cutting. 



BACKGROUND 



The last major breakthrough in coal 
mining cutting tool technology was in the 
1940's, when the introduction of tungsten 
carbide significantly increased the life 
and cutting ability of mechanical tools. 
Recent studies with water-jet-assisted 
cutting indicate that significant im- 
provements can be made to the cutting 
process by using this new technology. 

Using conventional cutting technology, 
today's coal mining machines, such as 
the continuous miner and longwall shear- 
er, have generally been optimized with 
respect to their cutting ability in re- 
lation to their size and weight. When 
cutting, these machines must accept high 
reactive torque and thrust forces. The 
ability to react to these forces is a 
function of the machine's weight and 

'Supervisory civil engineer, Pittsburgh 
Research Center, Bureau of Mines, Pitts- 
burgh, PA. 

^Project research supervisor, Boeing 
Services International Inc., Pittsburgh, 
PA. 



tractive effort. Attempts to increase 
thrust and torque have resulted in in- 
creased machine weight and size, thereby 
decreasing machine maneuverability and 
productivity. 

The potential for a major advance in 
cutting technology has been indicated by 
researchers using water-jet-assisted cut- 
ting systems. Experiments have been con- 
ducted in a wide variety of rock types 
(both in the laboratory and underground) 
that demonstrate substantial improvements 
in cutting performance along with sig- 
nificant improvements in health and safe- 
ty. These improvements include signifi- 
cant reductions in pick cutting and 
normal forces; improvements in bit life, 
with substantial reduction in failures of 
the tungsten carbide inserts; reduction 
in fines; significant reductions in dust; 
and reduction of frictional ignitions. 

3 Industrial hygienist, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, PA. 



22 



APPROACH 



The Bureau of Mines is currently pur- 
suing a program to develop a water- 
jet-assisted rotary cutting system that 
can be used for a longwall shearer or a 
continuous miner. This system will use 
water-jet pressure ranging from 3,000 to 
9,000 psi, without increasing current 
water usage or significantly increasing 
total power consumption. Research has 
shown that water-jet-assisted cutting has 
the potential to reduce machine thrust 
and torque when compared to conventional 
cutting. The limits of this study are 
shown in figure 1 bounded by the shaded 
area. Previous researchers explored this 
new technology through laboratory testing 
of coal and coal measure rock for frac- 
ture properties but fall short of the 
desired goal to predict mining machine 
performance, component wear, and costs. 
The approach adopted here was to use 
an In-Seam Tester to find the relation- 
ships between dry and water-jet-assisted 



UJ 

o 




fl 



2.5 3 4 5 6 7 8 9 10 
PRESSURE, I0 3 psi 
FIGURE 1. - Water-jet study range of force and 
ow rate parameters. 




FIGURE 2. - Heavy-duty In-Seam Tester. 



23 



cutting in coal and coal measure rocks. 
The In-Seam Tester (fig. 2) is a hydrau- 
lically powered single pick that cuts up- 
ward in a linear plane at 25 ft/min. The 
pick is instrumented with a four-pillar 
dynamometer to measure and record the 
orthogonal cutting forces as illustrated 
in figure 3. 

The rationale for using an In-Seam 
Tester is that one of the major diffi- 
culties facing investigators conducting 
laboratory coal cutting tests is to en- 
sure that the relatively small samples 
being cut are representative with re- 
spect to the confined ground-induced 
stresses. This is more of a problem with 
coal than other rock types because of 
cleat and joint presence in the coal mass 
and dehydration and temperature changes 
that occur when blocks of coal are 
removed from the mine. The In-Seam Test- 
er will allow use of full-scale tools to 
avoid potential errors in force scalar 
relationships . 

Cutting tests were made on coalcrete, 
underground in the Pittsburgh coal seam, 
and on blocks of limestone and sandstone. 
Properties of these materials are shown 
in table 1. Coalcrete (cement, flyash, 
and coal) is a synthetic coal block that 
is cast in place to simulate coal 
properties. 

Two cutting tools were tested: the 
longwall flat pick and the conical bit 




FIGURE 3. - Force reactions on a cutting bit. 

(fig. 4). Each cutting tool was tested 
with and without water-jet assist using a 
front-mounted water jet that impinges 
directly in front of the cutting tool us- 
ing three pressure ranges: 3,000, 6,000, 
and 9,000 psi. 

The orthogonal bit forces were measured 
with a four-pillar dynamometer that was 
designed and fabricated at the Pittsburgh 
Research Center. One inherent feature of 
the dynamometer is that it is more sensi- 
tive to side and cutting forces than it 
is to normal force. 

The water-jet nozzles were made from 
stainless steel, 3/8-in-diam hexagonal 
socket head screws, which were machined 
down to an internal Leech and Walker con- 
figuration. Three nozzle exit diameter 
sizes (0.6, 0.8, and 1.0 mm) were used 
(fig. 1). 



TABLE 1. - Material properties and test results 



Item 



Density lb/ft. 

Porosity pet. 

Unconfined strength, psi: 

Compressive 

Shearer 

Hardgrove grindability index 

Cutting force with flat bit, lbf: 

Dry 

Wet ] 

Cutting force reduction pet. 

Depth of cut in. 

Bit spacing in. 



Berea 
sandstone 



Indiana 
limestone 



Coalcrete 
block 



Pittsburgh 
seam coal 



130 
1.98 

8,300 

NA 
NA 

3,620 

1,695 

53 

1 

2 



145 
14.1 

4,700 

NA 
NA 

NA 

NA 

NA 

1 

2 



106 

NA 

898 

132 

62 

350 

380 

Neg 

1.5 

3 



85 

NA 

NA 
NA 
58 

450 
390 
Neg 
1.5 
3 



NA Not available. Neg Negligible. 

^sing 3,000-psi water jets with 0.015-in-diam jet nozzle. 



24 




FIGURE 4. - Flat and conical cutting bits. 



Each cutting test with the In-Seam 
Tester consisted of data from 140 in of 
rock-coal cutting. The cutting stroke on 
the Tester is 14 in. Each cut had 10 in 
of steady-state cutting data recorded. 
The typical cutting sequence progresses 
from left to right with 3-in spacing 
between each cut. The depth of cut 
ranged from 1 to 2 in, depending on the 
spacing and cutting material, as shown in 
figure 5. After each test, a cleanup cut 
was made, progressing from right to left, 
using half spacing that cut between each 
groove of the previous cut. 

Dust measurements for dry and wet cuts 
were made during operation of the In-Seam 
Tester. Dust levels were measured during 
cutting by drawing air near the cutting 
tool through a duct connected to a dust 
box. A vacuum pump was used to draw dust 
from the cutting surface into the duct 
and through the dust box. Two cyclones 
were placed inside and near the middle of 
the box, connected to tubing that carried 
dust from the cyclones to sampling in- 
struments located outside the sampling 
box. 



1.5- in depth 
of cut typical 




FIGURE 5. - Cutting sequence of coal face. 



25 



RESULTS 



Water-jet-assisted cutting is the syn- 
ergistic combination of a mechanical tool 
and a directed high-pressure water jet. 
The high-pressure jet exploits any cracks 
in the vicinity, lubricates the cutting 
process, increases bit life, and reduces 
dust and incidence of frictional igni- 
tion. Water jets are especially effec- 
tive when cutting a granular texture like 
sandstone. 

Tests conducted on Berea sandstone, 
with properties as shown in table 1, dem- 
onstrated a 53-pct reduction in net cut- 
ting force and a peak force reduction of 
39 pet when using a conical bit and a 
0.015-in-diam jet nozzle with 3,000-psi 
pressure. These tests were conducted at 
a 1-in depth of cut and 2-in spacing and 
compare very favorably with reported 
results. 4 

When cutting softer materials such as 
coalcrete and Pittsburgh seam coal, the 
spacing was increased to 3 in and the 
depth to 1-1/2 in to provide higher force 
levels. The flat pick assisted by a 
3,000-psi jet indicated negligible force 
reductions. When cutting with the coni- 
cal bit, force reductions of 15 pet were 
observed. The conical bit was far more 
susceptible to the influence of 3,000-psi 
water jet than the flat longwall pick. 
As shown in table 2 , the conical bit re- 
quired twice as much cutting force as the 
flat pick. Tests in the coalcrete block 
and Pittsburgh seam coal clearly demon- 
strated that both materials exhibited 
negligible cutting force reductions with 
the 3,000-psi jet assist. 

The cutting of limestone with the coni- 
cal bit and 3,000-psi water indicated a 
30-pct reduction in normal force and 17- 
pct reduction in cutting force for a net 
resultant reduction of 24 pet. This 
agreed favorably with Ropchan's work. 5 
Rock chip distribution studies were 

^Ropchan, D., F. D. Wang, and J. Wolga- 
mott. Application of Water-Jet-Assisted 
Drag Bit and Pick Cutter for the Cutting 
of Coal Measure Rocks (U.S. Dep. Energy 
contract ET-77-a-01-9082, CO Sch. Mines). 
Apr. 1980, 120 pp.; NTIS DOE/ET/ 12463-1. 

5 Work cited in footnote 4. 



TABLE 2. - Bit cutting force reactions 
in coalcrete (l-l/2-in penetration 
and 3-in spacing) 



Cutting force, lbf: 
Dry 

With water-jet 
assist 

Force reduction. .pet, 




Flat long- 
wall bit 

360 

370 
Neg 



Neg Negligible. 

conducted on the limestone tests to eval- 
uate cutting efficiency of the conical 
bit and flat pick. As shown in figure 6, 
the flat pick produced more larger chips 
at greater depths of cut than the conical 
pick and is therefore more efficient. At 
more shallow depths of cut the conical 
bit is more efficient. 

To verify this observation, cutting 
forces were compared for dry and water- 
jet cutting on all of the test materials. 
It was found that the flat longwall pick 
used only 40 pet of the cutting force re- 
quired by the conical bit at the same 
depth of cut and bit spacing. 

Testing with 6,000-psi water-jet assist 
resulted in substantial force reductions 
when using the flat and conical picks, 



Standard 
sieve size 



Extrapolation 




o Flat bit, f/ 2 -in DOC 



''8 1 



• Flat bit, l-in DOC 

a conical bit, l/g-in DOC 

^Conicalbit, 1-inDOC ^ N O.^O.I8 9 5in) 



I in 



3/b 



No.16 (0.0469 in) 
20 40 60 80 100 

CUMULATIVE PERCENT PASSING 
FIGURE 6. - Rock distribution study. (Numbers 
at right represent standard sieve sizes; DOC = depth 
of cut.) 



26 



indicating a strong susceptibility to 
higher jet pressures. In addition, a re- 
markable decrease in the magnitude and 
number of peak cutting forces was evident 
as shown in figures 7 and 8. These fig- 
ures show typical cuts taken with the In- 
Seam Tester when cutting the synthetic 
coal block with the flat and conical pick 
both dry and with 1,000 and 2,000 psi 
water-jet assist. The number and magni- 
tude of peak forces were reduced dramat- 
ically when using water-jet assist, which 
makes for a much smoother cutting process 
that will increase machine reliabil- 
ity and life. These advantages compare 



Normal, with 1,000 psi water 



Cutting, with 1,000 psi water 



Normal, dry 



^Ayl^A/VvV^^^A^_ 



Cutting, dry 
FIGURE 7. • Flat-bit peak cutting forces. 



favorably with results reported by 
Ropchan. 6 

Cutting forces were reduced approxi- 
mately 35 pet when cutting coalcrete us- 
ing 6,000-psi water-jet assist with a 
flat bit (fig. 9). The interesting fea- 
ture of this curve is that the cutting- 
force reduction decreases as jet pres- 
sure is increased beyond 6,000 psi. The 
plausible reason for this is that at 
higher jet pressures the jet cuts a slot 
beyond the depth of the mechanical tool, 
which decreases the assistance to the 
chip fragmentation process. More data 
will be required to adequately explain 
this phenomenon. 

This same phenomenon is also true for 
the gross force reduction, which is the 
cumulative effect of the orthogonal cut- 
ting forces. As shown in figure 10, the 
force reduction decreases as jet pressure 
is increased beyond 6,000 psi. The same 
reason given above for reductions noted 
in cutting forces may also be applied 
here for gross force reductions. Again, 
more data will be required to explain 
this phenomenon. 

Before a water-jet-assisted rotary 
cutting system can be employed, a number 
of problems associated with the use of 
water jets at 3,000 to 10,000 psi need to 
be resolved for the equipment to func- 
tion. These problems include provision 
for delivering high-pressure water to the 
rotating drum (cutting head), protection 



^Work cited in footnote 4. 



Normal, with 2,000 psi water 



/AjWL 



Cutting, with 2,000 psi water 



Normal, dry 

Cutting, dry 
FIGURE 8. - Conical-bit peak cutting forces. 



Ld 

<r 
o 



04 

10 
5 20 
y 30 

Q 
Ld 

* 40 

50 

0123456789 10 

WATER JET PRESSURE, I0 3 psig 

FIGURE 9. - Flat-bit cutting force reduction 
while cutting coalcrete using 0.6-mm nozzle 
(correlation coefficient = 0.66). 



1a _ 

\ A 

^^^^ A ^ 

A "1" 

A 

4 i ♦ I T • 1 I I I 



27 



o 

3 
Q 
UJ 

cc 

LU 
O 
OC 
O 



UJ 

I- 
to 
> 
en 

CO 

to 
o 

ce 
o 



-iu 
0' 


i 


1 


i 


i 


i 


i 


i 




I 


10 


















- 


20 


















- 


30 








A 








A 


A 


40 






▲ 
















▲ 


▲ 


▲ 


50 
60 ( 


- A 
1 


1 


I 


I 


I 


I 


I 


I 


1 


) 1 


2 


3 


4 


5 


6 


7 


8 


9 K 



_ 2.0 



WATER JET PRESSURE, I0 3 psig 

FIGURE 10. - Flat-bit gross system force re- 
duction while cutting coalcrete using 0.6-mm 
nozzle (correlation coefficient = 0.76). 

of the nozzles from rock debris and other 
possibilities of mechanical damage, a 
method for quickly and easily replacing 
nozzles that become worn by erosion, and 
a system for phasing water-jet activation 
only when the bit is in contact with the 
cutting surface. This last provision may 
be necessary to prevent excessive water 
usage and to minimize the energy required 
to operate the jets. 

One major aspect of this study was to 
ensure that the total amount of water 
added to the cut coal did not exceed what 
is presently used on continuous miners 
and coal shearers. As shown in figure 
1 1 , the weight of water added to coal is 
inversely proportional to the machine 
mining rate. The current upper limit 
accepted by industry is approximately 1.3 
pet water for each ton of mined coal. 
The water-jet-assisted mining rate re- 
flects the water- jet-induced lower bit 
forces. However, the water-jet-assisted 
system does not include a phasing system, 



E 

4— 

o 

c 
o 

4— 

o 

Q. 

3 

of 

UJ 

5 



.5 - 



1.0 



Water-jet 
assisted 





30gal/min 

60 gal /mirf 

25 gal/min 
gal/min 
15 gal/min 



j_ 



j_ 



_L 



5 10 15 20 25 

MINING RATE,ton/min 
FIGURE 11. - Percent water in cut coal using 
various mining methods. 



100 
90 



1 


1 1 1 
^ — o 


i i i ■ ■■ i - — 





/ 




- 


- f*~ 


— Extrapolation 


- 


-1 


Reduction (pet) = 


Dust dry- dust wet 

7T— ; ( ioo) - 

Dust dry 


1 




- 


1 

1 1 


1 1 1 


i i i i 



80 



* 70 



o 60 
B 50 

UJ 

or 

t- 40 



30 



20 - 



10 



123456789 

WATER-JET PRESSURE, I0 3 psi 

FIGURE 12. - Water-jet-assisted dust reduc- 
tion potential. 

which could cut total water usage by a 
factor of two-thirds. With such a phas- 
ing system, the water could be reduced to 
approximately 0.50 pet per ton of mined 
coal. This observation was made using 
the 0.015-in-diam jet nozzle with 6,000 
psig and a flow rate of 1.2 gal/min per 
nozzle. 



28 



Water-jet-assisted cutting significant- 
ly reduces coal dust when cutting the 
Pittsburgh coal seam underground at 
the Pittsburgh Research Center (fig. 12). 



At 4,000 psi, dust reduction was 90 pet 
when compared to dry cutting. Beyond 
4,000 psi no further dust reduction was 
evident . 



CONCLUSION 



Four test materials have been evaluated 
with 3,000-psi water-jet assist. Al- 
though effective in coal measure rock, 
this jet pressure was ineffective in coal 
and coalcrete. Testing to date indicates 
that the flat pick is the more efficient 
cutting tool. Work has commenced using 
higher pressures. Results indicate that 



the 6,000-psi water jets will be effec- 
tive in coal and coalcrete. Though in 
the early research stage, the water- 
jet-assisted cutting program, which is 
being conducted at the Pittsburgh Re- 
search Center, has proved encouraging. 
It does warrant further study. 



29 



EXPERIENCE WITH BOOM-TYPE ROADHEADERS EQUIPPED WITH HIGH-PRESSURE 
WATER- JET SYSTEMS FOR ROADWAY DRIVAGE IN BRITISH COAL MINES 

By A. H. Morris 1 and M. G. Tomlin 2 



ABSTRACT 



In 1978, collaboration was established 
between the Bureau of Mines and the 
National Coal Board (United Kingdom) 
(NCB) on a project to develop and test a 
high-pressure water-jet system fitted to 
a boom-type roadheader. Earlier labora- 
tory trials had indicated that such a 
system could significantly improve the 
cutting performance of a roadheader and 
that other benefits in terms of extended 
pick life, better dust suppression and 
reduced frictional sparking would also be 
achieved. 

Surface trials with a prototype sys- 
tem showed that the expected benefits 
could be realized, and further equipment 



development was then carried out so that 
the potential of the technique could be 
assessed when applied in production 
situations. 

Four production-type water-jet systems, 
each incorporating different features as 
regards pump units, seals, nozzles, etc., 
have been produced and fitted to road- 
headers. The first of these has success- 
fully completed an underground trial, and 
trials with the others are proceeding. 
The experience gained with these ma- 
chines, the performance of the water-jet 
systems, and their effect on operation 
are discussed. 



INTRODUCTION 



Operating trials with a boom-type road- 
header equipped with the National Coal 
Board prototype high-pressure water-jet 
system were carried out at Middleton 
Limestone Mine in 1981. 3 Figure 1 shows 
the system in operation. During these 
trials , the technique showed good poten- 
tial for improving the roadheader cut- 
ting performance. It was agreed that 
the development of preproduction versions 



should proceed, so that full-scale un- 
derground trials could take place. Two 
machines would be produced by Anderson 
Strathclyde and Dosco, respectively, us- 
ing technical information and trial re- 
sults provided by NCB's Mines Research 
and Development Establishment (MRDE); two 
others would be Dosco Mk.IIA machines 
modified by MRDE. 



ACKNOWLEDGMENT 



The authors thank the Director of Min- 
ing Research and Development, National 
Coal Board, for permission to publish 
this paper. The views expressed are 

1 Chief project manager. 

2 Project engineer. 
Mines Research and Development Estab- 
lishment, Tunneling and Transport Branch, 
National Coal Board, Bretby, United 
Kingdom. 



their own 
the NCB. 



and not necessarily those of 



3piumpton, N. A., and M. G. Tomlin. 
The Development of a Water Jet System To 
Improve the Performance of a Boom Type 
Roadheader. Paper in 6th International 
Symposium on Jet Cutting Technology 
(Univ. Surrey, United Kingdom, Apr. 6-8, 
1982). BHRA Fluid Engineering, Crans- 
field, Bedford, United Kingdom, 1982, 
pp. 267-282. 



30 




FIGURE 1. - NCB prototype high-pressure water-jet system. 



DEVELOPMENT OF PREPRODUCTION VERSION 



GENERAL 

The high-pressure water-jet system op- 
erates at a maximum water pressure of 
10,000 psi. It comprises a high-pressure 
water pump and a rotary seal unit to in- 
troduce the water into the roadheader 
drive shaft and cutting head, the cutting 
head being equipped with nozzles to di- 
rect high-pressure jets into the area of 
the cutting picks. Operational experi- 
ence with the prototype system indicated 
that considerable further development 
of the rotary seal was necessary if an 
acceptable life expectancy were to be 
achieved in a production environment and 
that there was scope for improvement 



in nozzle design to produce more ef- 
ficient jets. The original roadheader 
equipped with the prototype system was 
retained at the limestone mine to gain 
further operational experience and for 
use as a test bed for new seal and nozzle 
developments. 

ROTARY SEAL DEVELOPMENT 

Development of the high-pressure water 
seal for the prototype system had been 
concentrated on the requirement to fit 
such a seal around the cutting head shaft 
as shown in figure 2. Further develop- 
ment based on this configuration resulted 
in an increase in seal life from 50 to 



31 



-Pressure cylinder 
SeaK \ ^Seal 




-Cutting head shaft 
FIGURE 2. - Rotary seal assembly. 



Cutting head shaft 
-Cutting head 



Layshaft gearbox 

Epicyclic gearbox 




Water passage-^ 1 — Small-diameter rotary seal 

FIGURE 3. - Section through layshaft gearbox. 



140 h, though this was unpredictable and 
still far short of the minimum accepta- 
ble life of about 500 h. A seal change 
was also a major operation, and machine 
downtime was excessive. Because of these 
factors, development was switched to 
two alternative approaches. Both incor- 
porate a small-diameter seal built into a 
cartridge assembly, which is readily ac- 
cessible and can be easily and quickly 
changed, thus reducing the importance of 
seal life as a limiting factor; also it 
was expected that seal life would dramat- 
ically improve as the diameter decreased. 

The first approach was to introduce a 
layshaft gearbox into the roadheader boom 
prior to the cutting head shaft (fig. 3). 
A gearbox was designed and manufactured 
(fig. 4) and extensively tested on the 
roadheader. It proved completely relia- 
ble in operation, and the seal unit could 
be changed in a few minutes. The small- 
diameter seal unit developed concurrently 
for this application also proved success- 
ful, and seal lives in excess of 200 h 
were achieved. 

The second alternative is shown in fig- 
ure 5. The roadheader boom has been mod- 
ified to accommodate an axial water pas- 
sage along its full length, with the 




FIGURE 4. - Layshaft gearbox. 



32 



Epicyclic gearbox- 



Boom trunk 



Cutting 
head 




Small -diameter 
Motor-, / rotary seal 




Water passage through motor, 
epicyclic gearbox,and boom trunk 

FIGURE 5. - Boom with axial water passage. 



3 xZ? 



Diam D 




FIGURE 6. - Tungsten carbide nozzle. 



Delrin 



Sapphire insert 
FIGURE 7. - Synthetic sapphire nozzle. 




small-diameter seal unit mounted at the 
end remote from the cutting head. While 
time restrictions prevented this method 
being tested in situ, it was felt that it 
had every chance of success. 

Both methods have been adopted in the 
preproduction systems described later. 

NOZZLE DEVELOPMENT 

A test program was initiated to assess 
the comparative performance of various 
nozzle types against that of the simple, 
synthetic sapphire orifices used on 
the prototype system. Nozzle performance 
was recorded on a test rig by measur- 
ing the pressure profile across the jet 
stream at various distances from the 
nozzle. 

The type of nozzle which gave best re- 
sults is shown in figure 6. Manufactured 
from tungsten carbide, this type has a 
highly polished inner face with a surface 
finish of 0.15 ym. Nozzles of similar 
shape, but made from stainless steel and 
with an inset synthetic sapphire orifice, 
as shown in figure 7, also gave good re- 
sults, although slightly inferior to the 
tungsten carbide type. 

Nozzle life and efficiency ratings 
could not be established during the test- 
ing period, and it was decided that both 
types should be incorporated in the pre- 
production versions. This would allow 
assessment of the useful life of each 
type by regularly checking the perform- 
ance of sample nozzles on the test rig 
and recording any fall off in efficency 
over long periods. 



DESCRIPTION OF PREPRODUCTION SYSTEMS 



NCB PREPRODUCTION VERSION TYPE 1 

The type 1 system closely resembles the 
prototype system used at Middleton Lime- 
stone Mine, with some refinements based 
on test results and further development 
work. 

The base machine is a Dosco Mk.IIA 
roadheader as shown in figure 8. High- 
pressure water is generated by an oil- 
water intensifier mounted on the road- 
header boom. Maximum output from the 
intensifier is 45 L/min at 10,000 psi 



pressure, and it is powered by a hydrau- 
lic swashplate pump unit at the rear of 
the machine. This pump unit is fitted 
with a remote control system to allow the 
machine operator to select either high- 
pressure water output from the inten- 
sifier or low-pressure output for dust 
suppression purposes if high-pressure jet 
assistance is not required. 

A layshaft gearbox with small-diameter 
seal unit is fitted into the boom to ac- 
cept the water output from the intensi- 
fier. The cutting head is equipped with 



33 




FIGURE 8. - NCB preproduction version, type 1. 




FIGURE 9. - Cutting head. 



34 



tungsten carbide nozzles. These stand 
clear of the cutting head body to mini- 
mize the standoff distance from nozzle to 
pick tip and are protected from external 
damage by steel shrouds, as shown in fig- 
ure 9. 

The water-jet system is interlocked 
with the roadheader electrical and hy- 
draulic circuits for "operator" protec- 
tion. Prestart warning is given by low- 
pressure drenching sprays directed at the 
cutting head, while the high-pressure 
jets cannot be switched on unless the 
cutting head is running. 

NCB PREPRODUCTION VERSION TYPE 2 

The type 2 preproduction version is 
shown in figure 10. The only difference 
between the type 2 and type 1 systems is 
that, for the type 2, the single intensi- 
fier mounted on the roadheader boom has 
been replaced by two smaller intensifiers 
of equivalent capacity. These are mount- 
ed on each side of the boom, as shown in 
figure 11, to improve the machine opera- 
tor's view of the cutting head. 

DOSCO PREPRODUCTION SYSTEM 

The Dosco system is again based on 
a Dosco MK.IIA roadheader, which has 
been modified to accommodate the high- 
pressure water pump on the machine base 
(fig. 12). Some machine components have 
been interchanged, to enable a 150-kW 



through-shaft motor to be mounted at the 
side of the machine. This motor is then 
used to drive both the roadheader hydrau- 
lic pump and the high-pressure water 
pump. The pump used is a diaphragm-type, 
fixed-displacement unit with a maximum 
output of 45 L/min at 10,000-psi pres- 
sure. A water header tank is mounted 
above the pump , and the hydraulic circuit 
is arranged so that surplus output can 
be dumped back to this tank when high- 
pressure water is not required. 

A layshaft gearbox with small-diameter 
seal is fitted into the boom, as on the 
NCB systems, and high-pressure water is 
fed to this via a rigid steel pipe sys- 
tem; horizontal and vertical articulated 
joints accommodate movement of the road- 
header boom. The cutting head is similar 
in design to those used on the NCB sys- 
tems , except that it is equipped with 
synthetic sapphire nozzles. 

ANDERSON STRATHCLYDE 
PREPRODUCTION SYSTEM 

This system has been installed on an 
Anderson Strathclyde RH22 roadheader and 
is shown in figure 13. A triplex-piston 
type high-pressure water pump and water 
header tank are mounted on the rear of 
the machine behind the driver's position. 
This pump has a higher capacity — 68 L/min 
at 10,000-psi pressure — than the three 
systems previously described. 




FIGURE 10. - NCB preproduction version, type 2. 



35 




FIGURE 11. - Twin intensifiers mounted on boom. 



The roadheader is equipped with a boom 
incorporating an axial water passage 
and has a small-diameter rotary seal 
mounted at the motor end. The boom has 
a telescopic facility, and a sliding 
seal arrangement is incorporated in the 
water passage at the cutting head end. 
High-pressure water is fed to the boom 
by means of a flexible hose which, for 



reasons of safety, is shrouded by another 
heavy-duty hydraulic hose. The cutting 
head is based on the normal design used 
on the RH22 roadheader and has provision 
for high-pressure nozzles to be fitted in 
front of all 24 picks. As on the two NCB 
systems , the nozzles used are tungsten 
carbide. 



36 




FIGURE 12. - Dosco preproduction system. 
UNDERGROUND TRIALS 



GENERAL 

At the time of writing, the two NCB 
systems and the Dosco system had been 
commissioned and deployed to collieries, 
but trials had not yet commenced. Com- 
ments, therefore, are restricted to the 
performance of the Anderson Strathclyde 
roadheader, which has been in operation 
at Sutton Manor Colliery since August 
1983. 

Before taking the roadheader under- 
ground, operator training was carried out 
on the surface by MRDE and Anderson 
Strathclyde. Colliery staff were made 
familiar with the high-pressure equipment 
and its operation and were made aware of 
the hazards involved if the equipment was 
misused or if the rules established for 
safe operation were broken. 



The roadheader is being used to drive a 
main access roadway to coal reserves in 
the High Florida Seam. The roadway pass- 
es through a wide variety of strata, 
ranging from soft mudstones to hard silt- 
stones and sandstones with some ironstone 
bands and inclusions. So far, approxi- 
mately 150 m of drivage has been com- 
pleted with good results; the water-jet 
system is in continuous use and has been 
readily accepted by the heading teams. 
Colliery staff are satisfied that it is 
contributing significantly to the overall 
performance of the roadheader. 

CUTTING PERFORMANCE 

The comparative cutting performance 
of the roadheader with and without 
high-pressure jet assistance is being 



37 




FIGURE 13. - Anderson Strathclyde preproduction system. 



monitored in different strata as the 
roadway advances. The results of the 
tests completed to date are summarized in 
table 1. Increases in cutting rate and 
reductions in specific energy require- 
ments have been recorded in all cases, 
although there is considerable variation 
for each rock type. Examination of the 



recorder traces of the cutting head power 
consumption during tests also indicates a 
marked reduction in vibration when jet 
assistance is used. This was also vis- 
ually evident. Again, the effect has 
varied in the different rock types, but 
was extremely significant in some cases, 
as illustrated in figure 14. 



TABLE 1. - Comparative cutting performance with and without high-pressure 
jet assistance 



Test 


Rock type 


Strength, psi 


Cutting rate, 
ton/h 


Specific energy 
requirement , 




Compressive 


Tensile 


With 


Without 


m 3 /(kW«h) 




With 


Without 


1 




8,400 
6,300 
11,800 
10,200 
5,900 
5,900 
6,500 


2,200 
2,100 
3,000 
2,700 
1,700 
1,900 
1,700 


16.8 
17.1 
2.8 
7.8 
10.1 
37.4 
16.2 


15.2 

11.1 

2.5 

2.8 

8.1 

18.4 

13.0 


2.20 
2.23 
9.00 
2.87 
3.32 
1.21 
1.65 


3.58 


2 
3 




3.32 
10.27 


4 
5 


Siltstone and/or sandstone 


8.90 
4.09 


6 




2.46 


7 




2.30 



38 



With jets NL 


Without jets 

J ^ 





TIME 



DUST SUPPRESSION 

From the onset of the trial it was ob- 
vious that dust suppression with high- 
pressure jets was far superior to that 
achieved with conventional low-pressure 
(300-psi) sprays. This, in fact, has 
caused problems when monitoring the ma- 
chine cutting performance, since the op- 
erators are extremely reluctant to oper- 
ate without jet assistance even for short 
test periods. Continuous monitoring of 
respirable dust levels in the roadway 
over full working shift periods has shown 
that levels below 2 mg/m 3 have been con- 
sistently maintained, even though forcing 
ventilation is used. This is well below 
the maximum acceptable level of 5 mg/m 3 . 

The results of an individual series of 
tests carried out to establish the res- 
pirable dust make during the cutting cy- 
cle are given in figure 15. These tests 
were carried out consecutively in the 
same band of strata, and, by fitting 
suitably sized nozzles for each test the 
water flow of 68 L/min was kept constant 
over a range of pressures from 300 to 
10,000 psi. The benefits, in terms of 
dust reduction, of increasing the water 
pressure are apparent. 

PICK LIFE 

Pick consumption on the length of 
drivage already completed has averaged 
0.7 per meter of advance. Since jet 
assistance has been used continuously, no 
comparison can be made with pick consump- 
tion when jet assistance is not used. 
However, one short test has been carried 
out, and this indicates that considerable 
increase in pick life is being achieved. 



CT 



c/) 
Q 

Ld 

_J 
CD 
< 

or 



FIGURE 14. - Cutting head power versus time trace. — 



to 

e 




100 



WATER PRESSURE, 10* psi 

FIGURE 15. - Results of dust suppression tests. 



For the purposes of this test, two noz- 
zles were blanked off, one adjacent to a 
pick near the front of the cutting head 
and the other adjacent to a pick at the 
rear of the cutting head. During 1 m of 
advance , the front pick needed to be 
changed three times and the rear pick 
twice. Only 1 of the other 22 picks on 
the head failed during the same period. 
It was hoped that this test could be re- 
peated over a longer period so that accu- 
rate comparisons could be established; 
however, occasional sparking was noted 
during the test from the two picks with- 
out jets, and, in view of the gaseous 
conditions prevailing in the roadway, the 
test was not repeated. 

EQUIPMENT PERFORMANCE 

The high-pressure water system has op- 
erated well, and the only problems en- 
countered have been with the sliding seal 
in the telescopic boom. This has failed 
on three occasions but has since been re- 
designed and is now proving reliable in 
operation. 

Expected problems due to nozzle block- 
ages and water accumulation on the road- 
way floor (based on operational experi- 
ence with the prototype system) have not 
been as severe as expected. 



39 



The water feed to the high-pressure 
pump is filtered at 10 um through a bank 
of filters of diminishing size; normally, 
only one or two nozzles become blocked 
during a working week. Blockages can 
easily be cleared using a twist drill of 
the same diameter as the nozzle orifice. 

The floor of the roadway has remained 
generally dry, although some pools of 



water have accumulated in the working 
area. These do not affect maneuverabil- 
ity of the roadheader, though some prob- 
lems have occurred with the loading-out 
conveyors due to slurry causing block- 
ages. A pump has been installed in the 
roadway to clear the water if it becomes 
excessive. 



CONCLUSIONS 



Current trials with the water-jet sys- 
tem at Sutton Manor have proved that, to 
some degree, all the benefits expected 
from use of the technique, as evidenced 
by the first trials at Middleton, can be 
achieved. 

Cutting rates have been measurably im- 
proved, and the forces on the cutting 
picks have been reduced, as indicated by 
the lower specific energy requirement at 
the cutting head. 

The effect of high-pressure jets in re- 
ducing dust make has been remarkable, and 
this may well be a major factor in influ- 
encing further exploitation of the sys- 
tem, especially when considered in con- 
junction with the observed elimination of 
frictional sparking. 

Apart from early problems with the 
faulty sliding seal (which have since 
been solved), the high-pressure equipment 
has so far proved reliable in operation. 

What is also important is the way 
in which this new technology has been 



readily accepted by the colliery staff. 
Pretrial training has obviously paid div- 
idends in that the confidence of the col- 
liery staff has been gained; this exer- 
cise is currently being repeated at the 
other three trial sites. 

It would be premature to forecast the 
extent of future exploitation of high- 
pressure water-jet assistance, operation- 
al experience having not yet been gained 
with the other three systems. These will 
be operating in different conditions, a 
fact that may well highlight problems not 
apparent from the Sutton Manor trial. 
Trials with the NCB type 1 and Dosco sys- 
tems are now imminent, while trials with 
the NCB type 2 system will follow shortly 
thereafter. When these trials are well 
advanced, the overall performance of all 
four systems will be assessed. Only then 
will any commitment on further exploita- 
tion be made. 



40 



DEVELOPMENT WORK FOR COAL WINNING TECHNOLOGY 
By Dr. E. H. Henkel 1 



INTRODUCTION 



Worldwide, shearer-loaders and coal 
ploughs are the predominant machine de- 
signs for coal winning on longwall faces. 
Even though these systems are capable of 
high production performance under opti- 
mized operating conditions, they do not 
comply fully with tomorrow's expec- 
tations, which may be summarized as 
follows: 

1. Coal winning, including high per- 
formance, optimized web, and use on face 
ends. 

2. Adaptability to geological condi- 
tions, including seam thickness, strength 
of coal and adjacent strata, and working 
through faults. 

3. Working conditions including low- 
dust operation, improved mine climate, 
and higher safety standards. 

4. Economic aspects and cost reduc- 
tion, including coal preparation (larger 
particle size, less fines, and reduced 
humidity) , production (better use of en- 
ergy, higher reliability, and wider ap- 
plication range) , and organization (sys- 
tematic monitoring and maintenance). 

With consideration of these require- 
ments, the present limits of practicality 
for both types of coal winning systems 
are set as shown on figure 1. Inde- 
pendent of strength criteria, the appli- 
cation range for conveyor-mounted shearer 
loaders is in seam thicknesses of more 
than 1.8 m. For seam thicknesses of less 
than 1.8 m, in-web shearers may be used. 



Under the conditions prevailing in German 
coal mining, however, the performance of 
in-web shearers is unsatisfactory. 

The main application range of the 
coal plough is found in the lower seam 
thicknesses; however, coal plough opera- 
tions are limited to ploughable seams. 
Therefore, seams of hard coal and those 
within hard adjacent strata constitute 
a range for which, up to the present, 
cost-effective coal winning equipment 
for longwall faces has been nonexistent 
worldwide. 

These limits of practicality for 
shearer-loader and coal plough operations 
show that more concentrated energy trans- 
mission by the coal winning tools is re- 
quired for recovery of hard coal, selec- 
tive mining of rock, arriving at large 
webs, obtaining a better particle size, 
and eventually reducing the dust release. 

With respect to fulfillment of the 
above-specified requirements, the re- 
search for new technologies resulted in 
three candidate systems for coal winning: 

1. Use of high-pressure water jets. 

2. Use of hydraulic impact devices. 

3. Vibration transmission for fractur- 
ing the mineral. 

This paper discusses all three tech- 
niques, with special emphasis on the use 
of high-pressure water jets. 



ACKNOWLEDGMENTS 



The research and development work was 
sponsored by the Bundesminister fur For- 
schung und Technologie (Federal Ministry 

1 Head of Department and Test Side Coal 
Winning, Conveying, Face End Systems, 
Bergbau-Forschung GmbH, Essen, Federal 
Republic of Germany. 



for Research and Technology) as well as 
by the Minister fur Wirtschaft, Mittel- 
stand und Verkehr (Ministry for Economics 
and Transport) of Northrhine-Westf alia. 
The project was set up jointly by M.A.N.- 
GHH Sterkrade, Klockner-Becorit GmbH, 
Ruhrkohle AG, and Bergbau-Forschung GmbH. 



GENERAL TESTING PROCEDURES 



41 



For cost reasons, the development and 
testing of mining machinery is neces- 
sarily done in testing installations on 
the surface. However, one of the prob- 
lems of surface testing is realistic sim- 
ulation of operating conditions. This is 
accomplished with a simulated coal block 
that approximates the strength and rup- 
ture behavior in terms of required cut- 
ting forces. The volume of the mineral 
cut must also be comparable to that of 
the mineral encountered in actual mining 
operations. Since these problems have 
been dealt with in the literature, I pre- 
fer just to give a concise report on our 
experience. 

For industrial-scale tests, blends of 
marl, cement, and water are cast to ob- 
tain simulated coal. The setting time of 
these blends is approximately 3 months. 
Good results have been obtained vary- 
ing the components to obtain degrees of 
hardness. However, the requirement for 
more and more mockup coal faces necessi- 
tated shorter setting times, requiring 
simultaneous maintenance of controlled 
strength properties. Accordingly, addi- 
tives were proportioned to these blends, 



Medium 
70^db-50 



PLOUGHABILITY 

-Difficult 
50— /Jb*30mm 



Ab- 10 mm 




I 2 3 4 56789 10 
AV CUTTING FORCE OF A BIT(F S ) , kN 
FIGURE!. - Applications of coal winning machines. 



and these additives lead to shorter set- 
ting periods as a result of a lower 
water proportion of the blend. Neverthe- 
less, the problem of determining cutting 
forces, versus the volume of cutoff 
mineral, remained the same. The ratio 
cannot be established by determining 
compressive and tensile strength run 
on sample cubes of the same blend. For 
comparative assessment of cuttability of 
simulated coal face material and natural 
coal with dirt bands, we developed a cut- 
tability tester which measures cutting 
force and feed thrust. In this way a 
comparison of operating conditions can be 
made. However, the simulation of in situ 
rock pressures cannot be accomplished. 
Using the cuttability tester to assess a 
coal winning method for use in a particu- 
lar seam was practiced in approximately 
50 cases in the Ruhr district. 

The following are remarks regarding 
procedures followed in machine develop- 
ment on the testing premises of Bergbau- 
Forschung GmbH. Basically the work was 
performed in three phases. 

PHASE 1: TECHNICAL SCALE BASIC 
RESEARCH ON A NEW CUTTING TECHNIQUE 

Experimental investigations to assess 
the efficiency of any new cutting tech- 
nology and optimize parameters are run 
almost entirely by Bergbau-Forschung. In 
this phase, theoretical investigations 
carried out are of only a complementary 
character. If promising results are ob- 
tained, the second development phase is 
initiated. 

PHASE 2: CONSTRUCTION AND TESTING 
OF EXPERIMENTAL UNITS 

Application of a new technique means 
testing on the surface and underground. 
Accordingly, experimental units are fab- 
ricated with commercially available ma- 
chine elements and equipment. To allow 
assessment of coal winning capacity, 
the experimental units correspond as far 
as possible with a later prototype in 
terms of designed layout, drive rating, 
and traveling speed. In this development 



42 



phase, a manufacturer is sought who is 
interested in the new technique and who 
cooperates in setting up the experimental 
unit. Now the sometimes very long period 
of testing and modification on surface 
test rigs begins. If the results are 
positive, a colliery is located where the 
experimental unit can be tested under- 
ground. If possible, to allow ample 
opportunity for evaluation and modifica- 
tion, all underground testing should be 
conducted on nonproduction faces. If 
good results in terms of performance, re- 
liability, and maneuverability in under- 
ground operations are obtained, then the 
third phase, construction of the proto- 
type, is begun. 

PHASE 3: CONSTRUCTION AND TESTING 
OF THE PROTOTYPE 

Construction and, above all, testing 
of the prototype necessarily require 
cooperation of engineers from the manu- 
facturer, the colliery, and the research 
center. Only this cooperation enables 
a maximum of technology transfer to take 
place, and only in this way is it assured 
that the participating engineers be- 
come personally involved in the project. 
Without these people participating, the 
success of machine development becomes 
rather questionable. 

The new machine is designed, fabri- 
cated, and tested on the surface accord- 
ing to design parameters developed from 
experimental units. Later success in 
underground operations is to a consid- 
erable extent a function of the testing 
conditions on the surface, i.e., the 
quality of definition and simulation 
of the geological features and operating 
conditions. 



WATER-JET CUTTING TECHNOLOGY: 
OF DEVELOPMENT 



STATE 



During the recent past, several reports 
have been written about using high- 
pressure water jets for coal winning. 
The course of development leading up to 
the present is as follows: 



OPTIMIZATION OF CUTTING HEADS 

In the course of the basic investiga- 
tions, the cutting parameters of the 
water-jet technology were investigated 
for fixed and oscillating nozzles (fig. 
2) . The tests were run on a special test 
rig with "normally ploughable" mockup 
coal. For measuring the cutting forces, 
the cutting head was mounted in a special 
frame for triaxial measurements. For a 
pressure of 10,000 psi at the pump and a 
flow of 150 L/min, the following opti- 
mized cutting parameters were obtained: 



Nozzle diameter (d) mm.. 

Oscillating frequency (f) 

Hz.. 

Depth of cut (b) mm.. 

Tractive effort on the 
plough chain (F) kN. . 



1.65-1.95 

3-5 
220 

40 



Compared to mechanical ploughing the 
combined hard metal- jet cutting technique 
resulted in an increase of cutting depths 
of almost 100 pet at identical tractive 
effort on the chain (fig. 3). These re- 
sults were obtained for the range of coal 
defined as "normally ploughable." 

CONSTRUCTION OF AN EXPERIMENTAL UNIT 

For testing the cutting technology, an 
experimental unit was made up from exist- 
ing machinery components, namely, coal 
plough, guides and skids, 300-kW drum- 
shearer loader motor, two high-pressure 
pumps, and four cutting heads on each ma- 
chine side. One cutting head is arranged 
for horizontally cutting the floor. The 
experimental unit was pulled by an hydro- 
static motor at speeds adjustable between 
0.1 and 0.4 m/s (fig. 4). 

Subsequent to the tests on the mockup 
coalface, operation tests underground 
were run at the Lohberg colliery. The 
experimental face was worked at a thick- 
ness of M = 1.8 m and a length of L =50 
m. The coal strength may be categorized 
as "normally ploughable." 



43 



Slewing / -_"^> 

range | ~ z-% 
60° 




Winning direction 





215 kW 






Electric 
motor 


High- 
pressure 
pump 









800 bar 



High- 
pressure 
pump 



Oil 

pump 



f = 3Hz 



Electric 
motor 



FIGURE 2. - Jet miner, principle diagram. 








20 60 100 140 180 220 260 300 340 380 420 460 

CUTTING DEPTH, mm 

FIGURE 3. - Comparison of mechanical cut- 
ting and jet cutting. 




FIGURE 4. - Jet-miner winning machine. 



The experimental operation yielded the 
following results: 

- The coal face was undercut smoothly 
by the oscillating high-pressure wa- 
ter jets and subsequently broken off 
by the hard-metal tools. 

- The cutting heads exhibited no wear, 
and in particular, no wear attributa- 
ble to mechanical cutting work. 

- The measured chain force, F = 340 kN, 
shows that the power requirement of 
the high-pressure pumps could be re- 
duced from P = 300 kW to P = 220 kW, 
and that the water flow could be re- 
duced from 226 L/min to 170 L/min. 

- At a depth of cut of b = 400 mm, a 
traveling speed of v = 0.4 m/s was 
obtained. 

- The raw coal exhibited an excellent 
particle size distribution. 

- The quantity of dust released corre- 
sponded to approximately 30 pet of 
the dust produced by a shearer oper- 
ating in the same seam. 

DEVELOPMENT OF THE JET-MINER CONCEPT 

After successful trials run with the 
experimental version, the development of 
the first Jet-Miner prototype was started 
in cooperation with Ruhrkohle AG, M.A.N.- 
GHH-Sterkrade, and Bergbau-Forschung 
GmbH. Since the basic suitability of the 
oscillating-water-jet system was proved 
throughout the trials , the original con- 
cept and design features of the cutting 
heads were retained. As mentioned in the 
introduction, the Jet-Miner was supposed 
to be used in seam thicknesses of 1.0 to 
1.5 m. Accordingly, a machine body of 
1,000 mm height was designed. An array 
of two or three cutting heads ranging in 
height from 1.0 to 1.5 m was designed for 
the machine. 

During the underground trials , clearing 
problems arose, resulting in the total 
machine body of the Jet-Miner being situ- 
ated in the space between the conveyor 



44 



and the coal face. (The conveyor is 
straddled by a portal structure so that a 
good clearance profile is assured.) To 
eliminate problems of energy and water 
supply, a special cable guidance system 
was developed which is supposed to allow 
trouble-free supply via cables and hoses 
even at traveling speeds of 0.5 m/s. 

The prototype built by M.A.N.-GHH- 
Sterkrade, subsequent to elaboration of a 
joint concept, exhibits the following de- 
sign data: 

Length m. . 8.56 

Height mined .m. . 0.98 

Pump drives kW.. 215 

Water flow L/min. . 162 

Water pressure MN/m. . 70 

Total mass kg.. 18,000 

SURFACE TRIALS 

The Jet-Miner prototype is designed for 
coal winning underground. For checking 
the overall system feasibility, a surface 
testing program was carried out. Except 
for the length, the surface test dupli- 
cated all machinery components that were 
used underground. A 35-m-long test rig 
was set up in the test facility for coal 
winning and coal clearance techniques at 
Bergbau-Forschung GmbH (fig. 5). The rig 
operates on a mockup coal face 13 m long 
and 1.5 m high. This mockup coal face 
had a strength corresponding to the re- 
spective values of coal prevailing in the 
production face envisaged. 

The surface trials had the objective of 
testing the haulage system, the hydraulic 
drive system, the electric control in- 
stallation, and extraction output and 
loading trials. The cutting test result- 
ed in a winning performance 50 pet higher 
than with the first test rig. The 
loading capacity could also be improved. 
When passing horizontal and vertical un- 
dulations, the unit showed good running 
properties. 

UNDERGROUND TRIALS 

At Lohberg colliery seam R was prepared 
for the test under production conditions. 
The existing plough system had been re- 
placed by the Jet-Miner. The coal face 
was 260 m long with a seam thickness of 



1.45 m. The Jet-Miner was designed in 
such a way that it could run on any ex- 
isting plough guides. Only the special 
guidance system for energy and water sup- 
ply was custom-fitted. 

Only components cleared for underground 
operation were used. The hydrostatic 
drive system used nonflammable fluids. 
The conveyor, Jet-Miner, machine haul- 
age, and high-pressure pumps were con- 
trolled from a central control consol us- 
ing an audiofrequency system supplied by 
Siemens. 

All equipment used by Bergbau-Forschung 
was first cleared for underground opera- 
tion. The following parameters are mea- 
sured and recorded by UV-recorders and 
magnetic tape systems: Hydraulic pres- 
sure of the hydrostatic drives (measure 
for chain traction effort) , and speed of 
the drives (traveling speed). 

The underground tests yielded the fol- 
lowing results: 

1. The tractive efforts on the chain 
reached maximum values of more than 600 
kN. 

2. The Jet-Miner exhibited a strong 
climbing tendency. 

3. The cutting of the roof and of dirt 
bands was unsatisfactory. 

4. The mechanical elements, e 
high-pressure pumps, the hoses, 
dilating mechanism for the nozzles, the 
mechanical horizon control, etc., had 
high failure rates. 

The evaluation of these results leads 
to the following conclusions: 

1. The high tractive efforts on the 
chain were due to — 

A. Excessive friction within the 
plough guide system and corresponding 
high wear. 

B. Unsatisfactory cutting head 
performance due to insufficient water 
jet pressure on the nozzles due to 
clogging of the nozzle oscillating 
mechanism. 

2. The reason for the strong tendency 
to climb is the arrangement of the lower 
cutting head in a 30° position from the 
vertical. 

3. The controls of the hydrostatic 
drives worked unsatisfactorily in terms 
of synchronization so that uncontrolled 
chain slackening occurred. 



g., the 
the os- 



45 



/ / 




FIGURE 5. - Jet-miner test facility. 



46 



PRESENT SITUATION 

Following the underground tests, the 
results were evaluated and an extensive 
investigation of cutting heads was con- 
ducted. As a result, several modifica- 
tions were made to improve cuttability at 
long nozzle-face distances by nozzle de- 
sign, water conditioning, smoothing of 
the high-pressure flow upstream of the 
nozzle, improved water supply, and modi- 
fication of the oscillating system. 

Figure 6 shows the optimum pressure de- 
veloped for various nozzle designs com- 
pared with the data from the original wa- 
ter supply system. When using curved jet 
tubes, the oscillating system, however, 
causes, almost inevitably, a strong pres- 
sure drop at larger standoff distance. 

PERFORMANCE SPECIFICATIONS 

When designing the Jet-Miner the fol- 
lowing performance specifications should 
be met according to present knowledge: 

1. Cut over the total worked thick- 
ness, including roof and floor. 

2. Establish horizon control by ad- 
justment of the floor cutting system. 

3. Provide yaw control for the cutting 
heads. 

4. Provide retractability of the 
trailing head from the face. 

5. Load cutoff mineral in wide-web 
cuts at low traveling speeds. 

6. Guide machine over 
conveyor. 

7. Prevent clogging 
and plough guides. 

8. Provide improved hydrostatic drive 
control. 

THE DEVELOPMENT OF 



snaking 
of cutting heads 



o 



70 



60 - 



50 



LlI 

en 

Z) 
CO 
CO 

£ 40h 

Q_ 
CD 

z 30 

I- 

u 20 



10 



la 


1 


1 




~2a , 






- 


lb 


• ~». ^V 






-la— ^ 
2b-- 




N>--^^ 


— ,. 


— 


N 




— 


lb — 


\ ^v. 








"^0\ 




— 




\ ^ 


— 






\ 


^^*" 






\ 






KEY 


\ 




- / Drawn 


tube, 


\ 


\ - 


300 mm long 


a Optimized 




2 Curved tube 

i 


b Original 

1 









50 



100 



150 



STANDOFF DISTANCE, mm 

FIGURE 6. - Cutting forces of various jet 
tubes (tube 7 mm, nozzle 1.95 mm). 

At present, tests are being run on 
mockup coal faces in an attempt to meet 
these sometimes contradictory performance 
specifications. The tests mainly concen- 
trate on loading properties, cuttability, 
horizon control, and clogging. Another 
aspect concerns the reliability of ma- 
chine elements, accessibility to the in- 
dividual components, and monitoring and 
controlling the integrated system, name- 
ly, the plough, the conveyor, the drives, 
etc. 



THE IMPACT PLOUGH 



In the past, the function principle of 
the pneumatic pick, i.e., mechanical cut- 
ting by impact tools, turned out to be 
extremely efficient. The successful use 
of hydraulically activated percussive 
hammers, generally known under the term 
"impact ripper," in road headings, leads 
to the consideration of this technology 
also for coal winning machinery. Figure 
7 shows the function principle of an im- 
pact plough. According to the direction 
of travel, the leading impact hammers 



of a plough system are activated from a 
powerpack (P = 200 kW, pressure supply 
p = 2,000 psi) with an impact frequency 
of 5 to 10 Hz. 

The impact hammers assure a high 
corapressive-stress buildup at the tip of 
the pick. Accordingly, stone can be cut 
and faults can be worked through. Fur- 
thermore, the system can cut wide webs. 
For experimental use and testing of this 
technology, an experimental unit (fig. 8) 
was set up and tested on a mockup 



47 



coal face. The following problems were 
encountered: 



1. Guidance of the tool at high trans- 
verse forces. 

Hammer array. 

Loading of the cutoff mineral. 

Machine guidance along the coal 



2. 

3. 

4. 
face. 

5. Unsatisfactory 
hammers. 



function of the 



After extensive testing, the reasons for 
the deficiencies were identified, and re- 
medial action could be taken. At present 
the experimental unit has been recon- 
structed, checked, and will be run in an 
experimental face underground. We expect 
this development to be successful. 



Hydraulic breaker 
8 Hz -1,500 J/ blow 



CCa: 



CCm 



Winning direction 



Hydraulic breaker 
6.5 Hz -1,200 J/blow 



Electric 



a 



Hydraulic 
pump 





rOH 



t: 



FIGURE 7. - Impact plough, principle diagram. 




FIGURE 8. - Impact hammer, experimental unit. 



48 



BASIC INVESTIGATIONS ON VIBRATION TECHNOLOGY 



At present, basic investigations are 
being run within Bergbau-Forschung in 
view of using vibrations to fracture min- 
erals. For these investigations a test 
rig was set up on which a cutting tool 
has been arranged and onto which vibra- 
tions are induced by a hydraulic ram 
(fig. 9). The cutting tool is mounted to 
a measuring frame which is pulled along 
the coal face by a chain. The hydraulic 
ram is activated by a powerpack via 
servovalves. The oscillation frequency 
can be adjusted between and 50 Hz. 
Furthermore, various wave forms, e.g., 
sinus, sawtooth, or rectangular, can be 
selected. 

The cutting tests are run on various 
tool designs and cutting angles as well 
as on mockup coal of varying strength. 
Cutting forces, feed thrust, and trans- 
versal forces are measured as assessment 
parameters for the cutting performance 
of this technology. From the cutting 
force plot on figure 10 it may be seen 
that in the case of a nonactivated tool 
(frequency = 0) a maximum tractive effort 
on the chain of Fs = 70 kN is recorded 
for a cutting depth of b = 100 mm and a 
cutting tool width of 300 mm. For an 
identical cut, the tool vibrating at a 
frequency of f = 20 Hz with an amplitude 
of a = 5 mm, a cutting force of Fs = 15 
kN was required. The cutting force re- 
duction corresponds therefore to a factor 
of 4 to 5. 



Feed thrust (Fq) 



-Vibration tool 



Cutting force (F s 




Tractive effort (Fk) 



Measuring frame 
FIGURE 9. - Test rig, vibrating tools. 



80 



^ 60 

CO 

Ld 

O 

<r 40 
o 

LL 

H 20 

O 



KEY 

Cutting depth = 100 mm 
Vibration amplitude = A 

h*-A=0 








10 



20 30 40 

FREQUENCY, Hz 

FIGURE 10. - Cutting forces of vibrating tools. 

These positive test results gave us 
reason to develop, in a first phase, a 
hydraulic system suited for underground 
testing. For the next development phase 
it is intended to set up an experimental 
unit for surface and underground testing. 



SUMMARY 



Looking at present requirements for new 
activated coal-winning machinery, the 
necessity for such developments is point- 
ed out. 

The pattern followed for the required 
basic investigations, the setup of ex- 
perimental units, and the construction of 
prototypes is discussed. 



The present stage of development and 
application of high-pressure water-jet 
cutting technology, of impact plough sys- 
tems, and of vibration technology is out- 
lined. Specific problems as well as ap- 
plication limits are explained.' 



49 



THE WATER JET PLOW 
By David A. Summers 1 



INTRODUCTION 



This meeting takes place on the longest 
day of the year. It is, therefore, time- 
ly as well as pertinent to begin by wish- 
ing you "Heddwych" from the UMR- 
Stonehenge, which was dedicated by a 
druid of Geffodd of the British Isles at 
sunset last night (fig. 1). 

This topic is pertinent because the 52 
rocks of this half-scale model were 
carved, in their entirety, by high- 
pressure water. The trilithons of the 
inner circle (fig. 2) stand some 16 ft 



high and are made of Georgia granite with 
a compressive strength of 30,000 psi. 
The rock was cut by water jets, at an 
average pressure of 15,000 psi at the 
University of Missouri — Rolla (UMR) . To 
complete the project on time, cutting 
rates at least twice those of convention- 
al cutting were required, and achieved. 
This is an interesting topic in its own 
right, and also one that is very perti- 
nent to the subject of the meeting today. 



ACKNOWLEDGMENTS 



This work was funded under contract to 
the Bureau of Mines and the U.S. Depart- 
ment of Energy. The detailed design of 
the head was largely the responsibility 
of Dr. Clark Barker of the University of 
Missouri — Rolla, while the construction 
was in large measure carried out by the 
staff of the Rock Mechanics & Explosives 
Research Center. This project was very 



much a team effort, with input coming not 
only from those of us on the project, but 
also from the Government agencies and 
substantially from the industrial con- 
tacts we had with people we visited who 
came to see the machine work. We are 
very grateful and happy to acknowlege 
that assistance and advice. 



EQUIPMENT OPERATIONAL LIFE 



Most water-jet-cutting research com- 
prises a series of tests that are of 
short duration and concentrate on the 
cutting ability of the water jets. Data 
on the problems apt to be encountered 
during the lifetime of the equipment are 
rarely presented. The construction of 
the UMR megalith required that the high- 
pressure equipment operate at a pressure 
of 14,500 psi for approximately 1,000 h. 
This pressure is some 50 pet higher than 
that normally recommended for the cutting 
of coal. To cut a smooth channel down 
through the rock, the nozzle must be 
fed forward after each pass across the 

' Curators' professor, Rock Mechanics 
and Explosive Research Center, University 
of Missouri — Rolla, Rolla, MO. 



surface. This requires that a 5-cm-wide 
slot be cut into the rock to ensure that 
the nozzle can move freely. To cut this 
slot, two 0.925-mm-diam jets were rotated 
at approximately 300 r/min. The swivel, 
located in an unsupported position at the 
top of the cutting lance, allowed this 
rotation, and the same unit was used 
throughout the cutting (fig. 3). Noz- 
zles, that cut within 1 cm of the surface 
lasted around 20 h, with the major fail- 
ure being because of the surrounding 
metal holder, rather than nozzle fail- 
ure per se. Nozzle quality was not, how- 
ever, consistent. Rubber hoses, rated at 
40,000 psi burst pressure, held up well 
in the early stages of the program, 
but after some 600 h, several had to be 
replaced. 



50 







FIGURE 1. - Overview of the UMR Stonehenge. 



This information is provided to show, 
in part, how far the reliability of the 
equipment has come since 1974, when the 
Bureau of Mines granted UMR its first 
contract on the Hydrominer program. Al- 
though the granite cutting unit ran for 



24 h at 14,500-psi pressure on June 16- 
17, 1984, at that time virtually no 
equipment was available to run at even 
10,000 psi on such a continuous basis. 
Within this past decade water-jet equip- 
ment has, however, come of age. 



THE UMR HYDROMINER 



The use of high-pressure water for cut- 
ting coal has many potential advantages, 
and a likely equivalent number of pit- 
falls for the unwary. In this brief 
presentation, it is not possible to cover 
all of these; such a discussion is given 
on the final report of the Hydrominer 
project CO. 2 

The Hydrominer is a water-jet-assisted 
plow. In the original concept of the de- 
vice (2^, water jets cut along the perim- 
eter of the plow, cutting an access slot 
for the wedge head to enter and wedge off 
the central coal core (fig. 4). Follow- 
ing laboratory tests undertaken to estab- 
lish the basic cutting parameters of the 

^Underlined numbers in parentheses re- 
fer to item in the list of references at 
the end of this paper. 



jet and to determine to what degree the 
initial design would be valid, a prelimi- 
nary unit design was developed, con- 
structed, and field-tested in a surface 
coal mine in the Midwest in 1976 (fig. 
5). One initial change that had to be 
made in that program was to ensure that 
all the water-jet nozzles oscillated over 
a given cutting path. In the original 
conception, it was felt that the jets 
would cut the required depth from a fixed 
position on the plow body. However, be- 
cause of the jet and slot interference 
and the blocking of subsequent jet action 
by the rebound of the preceding flow, 
fixed jets do not give the performance 
levels required. This work, since con- 
firmed by the failure of the fixed-jet 
plow in Germany O) , led to the simplifi- 
cation of the device into the form used 



51 




FIGURE 2. - 16-ft-high central trilithon, carved from granite by water. 



52 




FIGURE 3. - Swivel detail from granite cutting unit. 




FIGURE 4. - Artist's impression of UMR Hydrominer. 



53 



r // 




FIGURE 5. - Equipment on trial surface coal mine. 



in the field trial. The results of that 
trial were very encouraging, and every 
goal set by the Bureau for our perform- 
ance was exceeded. Potential advantages 
resulting from the use of the Hydrominer 
include — 

1. 40-pct increase in productivity at 
equivalent horsepower. 

2. Control over product size. — Note 
that larger coal means fewer losses 
and increase in preparation plant 
productivity. 

3. Elimination of respirable dust 
problem. 

4. Removal of the face gas ignition 
hazard. 

5. Ability to increase web depth with- 
out equivalent increase in horsepower. 

6. Reduction in haulage horsepower 
required. 



As a consequence of that study a sec- 
ond generation head was built; concur- 
rently an impetus was provided for the 
German study which has since developed 
(4). 

The effectiveness of a water-jet-mining 
machine, however, is critically dependent 
on making the best use of the jets that 
cut the coal. These jets only cut a slot 
around the perimeter of the mass being 
removed (fig. 6), a technique proven 
feasible by the widespread use, at one 
time, of the Meco-Moore mining machine. 
For most effect, this slot must run at 
least 4 in ahead of the plow blade. Such 
a slot depth cannot be maintained unless 
the jets oscillate over the face of the 
plow, but it also requires that the jets 
be generated from an effective nozzle 
design. 



54 




FIGURE 6. - Cutting pattern of slots for the Hydrominer. 



NOZZLE DESIGN 



The UMR group, and particularly Dr. 
Barker, spent considerable time improving 
nozzle design (_5_) . It should be clearly 
appreciated, from the outset, that coal 
does not behave in a similar manner to 
other rock under water-jet attack. In 
most rock, adjacent cuts can be made up 
to an intervening distance of perhaps 
three nozzle diameters before the inter- 
vening rib disintegrates; with coal the 
rib is removed with a spacing between 
cuts of up to 5 cm. 

The benefit that this gives is that it 
allows the use of a dual-orifice diverg- 
ing jet pair to cut slots in the coal. 
This in turn substantially improves po- 
tential cutting performance. A single 
jet cuts a slot of decreasing width as it 
penetrates the material (fig. 7). Thus 
on a second pass, over the same slot, the 
sides of the previously cut slot erode 



the sides of the cutting jet so that its 
performance is reduced. A reduction in 
performance of up to 50 pet may occur on 
this second pass, depending on the rela- 
tive efficiency and parameters under 
which each cut is made. If, in contrast, 
the flow is directed through two diverg- 
ing jets, the coal between these two jets 
will be removed as the jets make the pri- 
mary cut (fig. 8). Thus when the nozzle 
advances, and the second cut is now made, 
the jets do not meet the coal until they 
reach the back of the previous cut. In 
this manner the jets will cut the depth 
by which the unit has advanced, or the 
same depth as the first cut, depending on 
where the cut surface is relative to the 
nozzle. This relates to the decay in jet 
effective cutting pressure with standoff 
distance from the nozzle. 



NOZZLE POSITIONING AND CUTTING PATH 



It is important to locate the nozzles 
in the correct position on the plow head 
to achieve optimal cutting results. This 
pertains particularly to two aspects of 
the design, and leads to resolution of 
two potential problems. The cutting arms 



should be so oriented that they overlap 
in cutting to the full seam section, but 
concurrently the face angle jet should 
pass directly in front of the face cut- 
ting edge of the plow. This face should 
be a sharpened hardened metal surface, 



55 



Material 

removed on 

next pass 





FIGURE 7. - Slot cut by a single-jet nozzle. 



Material 

removed on 

next pass 




FIGURE 8. - Slot cut by dual-jet nozzle. 



with the intention that the jets will 
normally cut a path in the coal suffi- 
ciently wide that the head can enter the 
slot, with subsequent contact being made 
on the wedge section where the angled 
surface will cause the main coal column 
to break off from the solid. 

Where a layer of rock is present in the 
coal, the water jets will normally remove 
the coal from either side of the layer 
but will not always be able to totally 
remove the inclusion. Under these cir- 
cumstances, the synergistic effect of the 
water jet acting with a metal tool (the 
water-jet-assist mode as it is known) can 
be utilized through the combination of 
the plow edge and the oscillating jet. 
By such a means the Hydrominer was able , 
in its field trials, to cut through >3- 
in-thick lenses of pyrite. It is neces- 
sary to cover the full seam section, giv- 
en that one cannot predict where the 
bands of dirt will occur. 

The second important feature is to en- 
sure that the column of coal isolated by 
the vertical jets, cutting at the back of 
the plow, is also cut horizontally. Two 
different options were considered for 
this feature. In the original Meco-Moore 
design the column was broken at the bot- 
tom and half way up the section; this 
feature was changed, because of the 
smaller web which the Hydrominer takes , 
and also because in many coals there is a 
need to leave either a fixed amount of 
roof coal or a fixed amount of floor 
coal. A positive horizon definition is 
thus required, and therefore horizontally 
oscillating jets are placed to cut across 
the column at the roof and floor. This 
arm, however, does not have the major 
purpose of creating a cutting plane; 
rather the intent is to break the support 
column to the roof and thus transfer the 
abutment load from the coal, before the 
plow begins to fracture the coal, and 
load it onto the conveyor. Because of 
this feature the coal, at loading, is 
unstressed, and with no strength in ten- 
sion is easily broken into fragments and 
loaded onto the conveyor. Because of 
this feature the head becomes virtually 
web insensitive in terms of haulage and 



56 



cutting force. For example, in the 
trials at Moberley, the web was increased 
from 18 to 39 in with virtually no change 
in the haulage force required to move 
the machine down the face and to load 



the coal. The cutting force, through 
the jets, of course remains unchanged, 
given that there is no change in the jet 
system. 



DESIGN OF THE SECOND-GENERATION HEAD 



There were a number of substantial de- 
fects in the design of the first- 
generation head, which have been changed 
in the design of the second-generation 
unit. Apart from the need to substan- 
tially strengthen the head to resist the 
forces imposed in moving large tonnages, 
the most obvious feature was the decision 
to split the head into two modules. Each 
module contains a vertical oscillating 
arm and a horizontal oscillating arm 
(fig. 9) but is designed so that the two 
modules may be separated vertically by 
a set of hydraulic rams , to cope with 



variation in seam section (a feature we 
were not to pursue further in our devel- 
opment of this phase) . The modules can 
also be adjusted horizontally so that 
either the top or bottom section of the 
coal can be removed first. This feature 
is judged important since the lower force 
required to break out the coal will allow 
the top section of the coal to be removed 
first and then subsequently the lower 
section. This will allow the lower coal 
a free surface to lift up into, and thus 
will make loading of the coal from the 
face a much easier operation. 




FIGURE 9. - Second-generation cutting head. 



57 



The interior features of the head were 
simplified, in a manner detailed in the 
contract final report (1), so that a sim- 
ple robust head was constructed. The 
nozzles developed were of the more ad- 
vanced design and were fitted with guards 
to ensure that only a slot wide enough 
for the jets to escape existed on the 
head of the unit. Concurrently a flush- 
ing system was incorporated so that, at 
the end of each path, the jet struck a 



small metal surface which directed the 
jet back into the head, to flush any coal 
chips that had migrated back into the 
head out of the path of the arm. A sec- 
ond subsidiary circuit was built into the 
head, but not used, that would allow a 
low-pressure flow into the arm compart- 
ment to flush any large accumulation of 
coal out of the ports provided for that 
purpose in the design. 



REFERENCES 



1. Barker, C. R. , and D. A. Summers. 
The Development of a Longwall Water Jet 
Mining Machine (DOE contract AC01- 
75ET12542, Univ. MO— Rolla. July 1981, 
199 pp.; NTIS, DOE/ET/12542-T1. 

2. Summers, D. A. Water Jet Coal Min- 
ing Related to the Mining Environment. 
Paper in Proceedings of Conference on the 
Underground Mining Environment (Rolla, 
MO, Oct. 27-29, 1971). Univ. MO Press, 
1972, pp. 183-194. 

3. Henkel, E. H. , and T. Kramer. In- 
Seam Trials With the Hydrohobel. Paper 
in Proceedings of the First U.S. Water 



Jet Symposium (Golden, CO, Apr. 7-9, 
1981). CO Sch. Mines Press, 1982, 
pp. III-5.1 to III-5.7. 

4. Henkel, E. H. Development Work for 
Coal Winning Technology. See pp. 40-48 
of these Proceedings. 

5. Barker, C. R. , and B. P. Selberg. 
Water Jet Nozzle Performance Tests. 
Paper in the Fourth International Sympo- 
sium on Jet Cutting Technology (Univ. 
Kent, Apr. 12-14, 1978). BHRA Fluid En- 
gineering, Cranfield, Bedford, United 
Kingdom, 1978, pp. A1-A12. 



58 



DESIGN REVIEW OF JARVIS CLARK JETBOLTER 
By William C. Griffiths 1 



INTRODUCTION 



The transfer of technology from the 
world of research to the world of indus- 
try is always a challenging task, espe- 
cially when more than one organization is 
involved. In December 1982, Jarvis Clark 
saw the potential of the work being done 
in water-jet drilling by research organi- 
zations under Government sponsorship and 
entered into a collaboration agreement 
with Flow Industries, a leader in this 
field. We took the hardware developed by 
them and integrated it into the first 
custom-designed water-jet roof bolter. 
When we entered our collaboration agree- 
ment the extent of field testing of the 



water-jet-drilling concept was a 3-month 
underground test involving a retrofit of 
high-pressure water-jet componentry onto 
an existing conventional hydraulic rotary 
roof bolter. The results of this test 
were so encouraging that design work on a 
water-jet roof bolter began in March 
1983. 

The jetbolter is the final result of 
one year's intensive design effort to in- 
tegrate the high-pressure water-jet com- 
ponentry with the most up-to-date hydrau- 
lic and electric systems available to the 
coal industry. 



DESIGN CRITERIA 



The design starting point was a self- 
imposed set of physical constraints with- 
in which all of the componentry had to be 
packaged for both safety and protection. 
These constraints were — 

1. 24-in overall frame height to ena- 
ble future low-coal versions to be de- 
veloped with no chassis redesign. 

2. 9-ft maximum overall width. 

3. 24-ft maximum length. 

4. Provision for adjustable ground 
clearance and adjustable canopy. 

5. 5-ft maximum tramming height. 

6. Ability to drill a 6-ft hole in a 
single pass in openings 8 to 12 ft high. 

In addition to these physical con- 
straints, the design had to recognize, 
meet, and exceed all existing State and 
Federal safety requirements. 

Figure 1 shows the general layout with- 
in the single-piece box frame chassis and 
includes — 

1. The operator's compartment. 

2. The prime mover compartment con- 
taining electric motors and hydraulic 
pumps. 

1 General manager, Jarvis Clark Company 
Limited, Coal Division, Burlington, On- 
tario, Canada. 



3. The water reservoir. 

4. The hydraulic oil tank. 

5. A 320-f t-capacity , self-winding ca- 
ble reel. 

6. The electrical controller. 

7. The intensifier box. 

8. Four planetary geared, hydraulical- 
ly powered wheel units for tramming. 

This part of the machine should remain 
unchanged irrespective of the seam height 
the unit operates in. The Automatic Tem- 
porary Roof Support System (ATRS) booms 
and feeds will, of course, vary with seam 
height, although their basic design and 
mode of operation should remain similar. 

Figure 2 is a simplified schematic of 
the hydraulic oil and water circuits on 
the machine , from mine water supply to 
drill nozzle. This circuit is duplicated 
for the other boom. 

Water enters the circuit through a re- 
verse-flushing strainer containing a mesh 
element, which filters out any coarse 
material in the mine water supply. Be- 
tween the strainer and the water tank is 
the first of the two 10-ym filters. The 
water reservoir has a 130-gal capacity 
and is of stainless steel construction. 
There are internal baffles and an exter- 
nal water level sight gauge. The tank 



59 





^-f clearance onATRS^^r 
for tramming 

ELEVATION VIEW 

FIGURE 1. - General layout of jetbolter. 



capacity is sufficient for approximately 
4 h of drilling; however, if necessary, 
the unit can be run with the water hose 
continually connected. A rotary-gear- 
type water pump delivering up to 5 gal/ 
min at 80 psi feeds the intensifiers to 
ensure they always work under a positive 
head. Excess flow from the water pump is 
diverted through an oil cooler back to 
the water tank. 

The prime mover for each hydraulic cir- 
cuit is a 50-hp explosion-proof ac motor 
with drive shafts at each end. The tram- 
ming and intensifier circuit is driven 
from the pump at the rear of the machine, 
which is a variable- volume pressure- 
compensated piston pump set to deliver 
20 gal/min at 2,800 psi. On the front of 
the motor is a smaller variable-volume, 
pressure-compensated piston pump set to 
deliver 17 gal/min at 1,500 psi. This 
supplies booms, feeds, rotations, and hy- 
draulic pilot signals. All four pumps 



Drill 
nozzle— oi/ 4^><f~ 



Drill rotate 
mechanism 



Water on -off 
valve 



Water 

Feed 

Rotate 



Sting pilot valve - 

Control valve — ^ 




Hydraulic 
pump — . 
X. 




Blowdown 
valve 



Hydraulic 
reservoir 

0— Water 

pump ^ er 

filter 




Water . \ Reverse- 
reservoir^ f| ushing 

strainer 



FIGURE 2. - Jetbolter hydraulic oil and water circuits. 



60 



draw oil from the main 150-gal reservoir 
through a 100-mesh suction strainer. The 
hydraulic system is filtered through five 
6-um filters , two in the pressure line 
and three in the return line. 

The hydraulic and water circuits meet 
at the intensifier box, which is mount- 
ed on the outside of the chassis between 
the wheels. This box swings out on 
hinges to afford easy access for mainte- 
nance. Inside the box both intensifiers 
are on the chassis wall one above the 
other, together with the second water 
filter. Both water filters have a 10-ym 
rating and have throwaway cartridges made 
of spun polypropylene. Each filter has 
an internal bypass system. 

The intensifier is the heart of the 
high-pressure system and is a stainless 
steel reciprocating device, comprising a 
double-acting cylinder with check valves 
at either end and a hydraulic switching 
device on top. This device is mechan- 
ically activated. 

The principle of operation of the in- 
tensifier is shown in figure 3. At the 
end of the piston stroke the chamber be- 
hind the small plunger on the right is 
full of low-pressure water. Oil then en- 
ters the chamber behind the piston at 
2,800 psi and during the course of its 



stroke increases the water pressure by 
the area ratio of the piston-plunger, 
which in this case is 13:1, hence 35,000- 
psi water. At the same time as water 
exits the chamber via the high-pressure 
check valve on the right at 35,000 psi, 
water enters the opposing chamber through 
the low-pressure check valve. The 
switching circuit mounted on top of the 
intensifier then reverses the oil flow, 
and the stroke is repeated. The intensi- 
fier operates nominally at 80 strokes per 
minute to give an output flow of approxi- 
mately 1-1/4 gal/min. In industrial ap- 
plications this intensifier model series 
has accumulated over 3 million pumping 
hours at pressures above 30,000 psi. 

The two high-pressure lines then pass 
through the frame wall to the attenuator, 
which is a 3-ft-long gun, drilled from a 
stainless steel cylinder with a 2-in bore 
and a 2-1/2-in wall thickness. This is a 
surge-dampening device which smooths out 
the pressure spikes to provide a continu- 
ous flow of high-pressure water. This 
unit is placed under the oil cooler be- 
tween the hydraulic oil reservoir and the 
cable reel. 

From the attenuator, water flows to the 
crossover circuit. This is a series 
of valves enabling the two intensifier 



High-pressure 
water out 



Jt 







B 



Water in 




Hydraulic 
oil out 



Hydraulic 
oil in 

\ 



Hydraulic 
oil in 

1 



^J 



Hydraulic 
oil out 

t 



Water in 

A 



yjM 




m 



nn High-pressure 
- , i if jji^ — i water out 



m 




FIGURE 3. - Double-acting fluid intensifier; high-pressure water is forced to left (A) and right (S). 



61 



outputs to remain independent or to cross 
over or to interconnect with each other 
to provide higher power operation to one 
boom only. Up to this point all high- 
pressure water has been contained in 
steel tubing, but from the cross-over 
circuit to the drill mast high-pressure 
hose is used. All high-pressure steel 
tubing is painted yellow and marked "HI 
PRESS WATER." 

The high-pressure water hose is a six- 
wire-braid multispiral hydraulic hose 
with a 3/16-in ID. Rated working pres- 
sure is 35,000 psi, and burst pressure 
is 60,000 psi. Fittings are crimped on 
with a special crimping device which 
virtually forms a cold weld. Hose ends 
and connectors use high-pressure coned 
and threaded connections which are rated 
up to 150,000 psi. As additional protec- 
tion for the hose from external damage, 
it is sheathed in regular high-pressure 
hydraulic hose. A yellow plastic sheath 
marked "HI PRESS WATER" is then shrunk- 
fit over the hose assembly to distinguish 
it from other hydraulic hoses. 

The hose is routed along the boom to 
the foot of the drill mast, where it is 
connected to the water swivel via a hy- 
draulically operated on-off valve. This 
valve is connected through a safety cir- 
cuit which only allows water to flow to 
the swivel when the mast is stung against 
the roof. The water swivel is located 
below the drill stem drive box and is 
thus protected from any falling debris. 
This water swivel contains the high- 
pressure rotary seal that allows the pas- 
sage of high-pressure water at drill stem 
rotation speeds of up to 500 r/min. Seal 
life at this speed is 40 pumping hours, 
which is approximately 2 weeks' drilling 
under normal conditions. 

Drill stem rotation is by means of a 
hydraulic motor and chain drive. This 
drive produces 40 ft*lb of torque at 
500 r/min. The roof bolt inserter drive 
is next to the drill feed at a fixed dis- 
tance of 6 in. After the hole is drilled 
and the stem is withdrawn, the whole mast 
assembly is hydraulically indexed by 6 in 
and the bolt is installed with the in- 
serter drive. This is also powered by a 
hydraulic motor through a direct drive 
system. The inserter drive produces up 



to 250 ft'lb of torque. Both hydraulic 
motors and feeds are interchangeable. 
This system allows hands-off drilling 
with no delays due to drill steel 
changes. Bolts are placed in the in- 
serter while the hole is being drilled. 
Feed is accomplished through a chain and 
sprocket drive, set to provide 500 lb of 
thrust. 

The drill stem is a proprietary form of 
high-pressure tubing. The drill bit is 
threaded onto the stem at one end; at the 
other end, the stem is threaded into the 
water swivel. The drill bit is similar 
to a conventional tungsten carbide drag 
bit with set screws set into the bit ma- 
trix. Each set screw has a sapphire 
jewel cemented into it with an orifice 
drilled in it. Orifice sizes are typi- 
cally in the 0.007-to 0.01 1-in range. A 
fine-mesh screen at the bit-stem inter- 
face helps protect orifice plugging from 
any contamination introduced into the 
system downstream of the filters. The 
sapphire jewels are normally provided 
predrilled and precemented into the set 
screws for easy replacement in the drill 
bit. 

At the top of the drill mast, just 
above the drill stem collar, is a rubber 
boot. This collects all of the water 
and cuttings from the hole and diverts 
them down a 1-in hose to the ground at 
the foot of the mast. This ensures no 
splashback and a clean environment for 
the operator. 

Safety was a major influencing factor 
throughout the design. One of the most 
important safety features is the stinger 
pilot valve. The actuator for the pilot 
valve is an angled plate on top of the 
mast which turns about a pivot point. 
When the mast is stung against the roof, 
this plate activates a hydraulic pilot 
valve, which in turn opens the safety on- 
off valve next to the water swivel. This 
prevents the water jets being activated 
whilst in a position where injury may oc- 
cur to a second operator or a bystander. 
When the pilot valve is fully activated 
the plate rests against the top of the 
mast, thus allowing the mast to carry all 
the load. When the safety on-off valve is 
open, the normal on-off valve at the 
operator's station can be opened and 



62 




FIGURE 4. - Jetbolter prototype. 



drilling can commence. As an additional 
safety feature this on-off valve is non- 
detented. There is also a manual over- 
ride switch for the safety on-off valve. 
This must be depressed at the same time 
the normal on-off valve is activated. 



This is used only to check whether all 
the jets are free and operating. There 
is a pressure gauge at the operator's 
station that reads intensifier oil pres- 
sure and thus has a direct correlation to 
water pressure. 



CONCLUSION 



The jetbolter prototype, shown in fig- 
ure 4, is now at a coal property in West 
Virginia ready to begin underground test- 
ing. In the surface drilling tests con- 
ducted in sandstone of about 12,000-psi 
compressive strength, some very encourag- 
ing results were obtained. 

One-inch-diameter, 6-ft-deep holes were 
drilled consistently at penetration rates 
of 18 to 20 ft/min; this translates to 
20 s per hole; which is about three times 
the rate for a conventional rotary drill. 
This was achieved with no dust and no wa- 
ter splashback, and was quiet enough that 



a conversation at normal levels could 
take place at the drill operator's sta- 
tion. During these tests over 350 ft was 
drilled with a single bit without any 
appreciable wear. 

The potential for realizing these bene- 
fits has been known for some time from 
the research done in this area. The jet- 
bolter is our attempt to make these bene- 
fits commercially available to the coal 
industry at a time when any potential 
productivity increase will realize major 
long-term benefits for the industry as a 
whole. 



63 



WATER- JET-ASSISTED TUNNEL BORING 
By Dr. Levent Ozdemir 1 




ABSTRACT 



This paper presents and discusses the 
results of a laboratory research pro- 
gram designed to investigate the perform- 
ance improvements that can be gained by 
employing low-pressure water jets to 
assist disk-roller-type cutters. The ob- 
ject is to enhance hard rock boring per- 
formance through the use of water jets to 
increase attainable penetration rates and 
to lengthen cutter life, thus reducing 



mechanical boring costs. The technique 
principally involves placing a low- 
pressure jet directly in front of the 
cutter with the jet impinging the rock 
surface along the cutter path. The re- 
sults obtained thus far indicate that wa- 
ter jets designed to enhance disk cutting 
performance in such a manner offer the 
potential for improved boring rates and 
reduced excavation costs. 



INTRODUCTION 



Mechanical tunnel boring technology has 
realized significant advances over the 
last two decades. A better understanding 
of rock failure mechanism coupled with 
invaluable knowledge and experience 
gained from case history studies has con- 
tributed to better design approaches with 
resultant improvements in machine per- 
formance and utilization. Today, tunnel 
boring machines (TBM's) are being widely 
used in various aspects of underground 
excavation with favorable economics. 
Their application to hard rock boring is 
steadily growing as improvements in cur- 
rent technology for machine design and 
fabrication continue. 

Despite all these advancements, how- 
ever, TBM's still have certain limita- 
tions for use in the excavation of very 
hard and abrasive rock formations where 
low penetration rates accompanied by high 
cutter wear generally reduce their effec- 
tiveness as a viable alternative to con- 
ventional drill and blast methods. Since 
no new major breakthroughs are antici- 
pated in TBM technology in the foreseea- 
ble future, one alternative to achieving 
further improvements in mechanical boring 
performance is to utilize an auxiliary 
assisting mechanism as an integral part 
of the machine. To this end, water jets 

1 Director, Earth Mechanics Institute, 
Colorado School of Mines, Golden, CO. 



show great promise for enhancing TBM 
performance. 

A significant amount of laboratory and 
field research has been carried out over 
the past several years to provide infor- 
mation for a comprehensive assessment of 
the technical and economic feasibility of 
using water jets to enhance TBM perform- 
ance. The majority of this work was un- 
dertaken by the Colorado School of Mines 
(CSM) and by Bergbau-Forschung GmbH of 
West Germany. The principal goal of 
these efforts was to develop a workable 
cutting system utilizing water jets in 
conjunction with disk cutters for tunnel 
boring applications. 

The research effort at CSM in water- 
jet-assisted cutting initially began with 
an extensive series of laboratory rock 
cutting tests using a large linear cut- 
ting machine. Small-diameter jets with 
pressures up to 50,000 psi were employed 
to create slots in the rock in front of 
or between the cutter travel paths. The 
objective was to establish "free faces" 
in the rock to promote rock failure and 
chip formation. Thus, the water jets 
were oriented to cut kerfs either between 
or along the cutter paths , and also in a 
combination of both. The results were 
impressive, showing significant improve- 
ments in the rock fragmentation effi- 
ciency of disk cutters with jet assist. 
To demonstrate the feasibility of the 



64 



developed technique under field boring 
conditions , a follow-on program was ini- 
tiated in which a 7-ft TBM equipped with 
disk cutters was modified to accept the 
water-jet nozzles together with all an- 
cillary equipment. A series of field 
tests were run in a hard granite quarry 
near Skykomish, WA, and a large amount of 
test data for boring with and without jet 
assist were gathered. Overall, a 50 -to 
60-pct increase in TBM penetration rate 
was observed when water jets were acti- 
vated to assist the cutting mechanism. 
For some tests, the water jets were found 
to increase the penetration rate more 
than twice. Although the field tests 
were successful, confirming the technical 
feasibility of water-jet-assisted cut- 
ting, several problems likely to arise 
from the potential application of this 
concept on TBM's were also identified. 
Aside from safety concerns, the long-term 
operational reliability of the pumping 
equipment necessary to generate the de- 
sired pressures posed serious questions 
about the feasibility of the system for 
field application. 

Following the Skykomish field trials, 
tests with similar objectives were also 
carried out by Bergbau-Forschung GmbH of 
West Germany. An 8.5-ft-diam Wirth tun- 
nel borer was equipped with high-pressure 
water jets, and tests were run in a sand- 
stone quarry. Several arrangements of 
jet locations with respect to the cutters 
were investigated. The pattern involving 
a more or less uniform distribution of 
jet nozzles over the cutterhead was found 
to produce the greatest benefit of jet- 
assisted cutting. In this case, the ma- 
chine thrust required to maintain a given 



penetration rate was reduced by about 57 
pet with jet assist as opposed to mechan- 
ical boring only. Moreover, the machine 
power requirements were reduced by 43 pet 
through the use of water jets. However, 
the beneficial effects of water-jet- 
assisted cutting were found to diminish 
at high rates of penetration. 

In spite of the fact that the applica- 
tion of high-pressure water jets has 
proven to be a technically feasible con- 
cept as a means to enhance TBM perform- 
ance in hard rock, the questions related 
to system reliability for extended peri- 
ods of operation under adverse field 
conditions have prevented its full ac- 
ceptance and utilization by the TBM manu- 
facturers. Another shortcoming of the 
developed concept was the requirement for 
relatively high power, which had to be 
added to the system to operate the water- 
jet pumps. Also the high jet pressures 
posed serious safety concerns. As a 
result of these potential constraints, 
it became apparent that the jet pres- 
sures would have to be reduced substan- 
tially to minimize power requirements, 
enhance equipment reliability, and im- 
prove personnel safety. These considera- 
tions eventually led to new research and 
development efforts with the objective 
of devising a low-pressure jet-assisted 
cutting technique for implementation on 
TBM's. In this aspect, several research 
programs have been conducted at CSM over 
the last 4 years. These included jet- 
assisted cutting in both soft and hard 
rock formations. In this paper, only the 
results of cutting tests in hard rocks 
are discussed. 



LABORATORY TEST RESULTS 



The laboratory test program for inves- 
tigating the effectiveness of water- 
jet-assisted disk cutting was conducted 
on a large linear cutting machine. Tests 
were performed using a constant penetra- 
tion mode of testing whereby cutter pene- 
tration into the rock was held constant 
and the cutter forces required to main- 
tain this penetration depth were measured 
and recorded. All three orthogonal cut- 
ter force components (side, normal, and 



rolling) were measured by a triaxial load 
cell and recorded using high-speed digi- 
tal integrators, as well as strip chart 
recorders. Various types of pumps, in- 
cluding axial-piston and triplex pumps, 
were employed to supply the necessary jet 
pressures and flow rates. 

As noted previously, the jet nozzle for 
the linear cutting tests was positioned 
directly in front of the cutter and ori- 
ented to impinge the rock surface along 



65 



the cutter path. The jet nozzle was se- 
curely attached to a fixture designed to 
permit setting of different standoff dis- 
tances and jet orientation angles. 

As part of various research investiga- 
tions, jet-assisted disk cutting tests 
were carried out in samples of two rock 
types, granite and basalt. The granite 
samples were those of Colorado Red Gran- 
ite, which has an average compressive 
strength of 21,000 psi. The basalt sam- 
ples were obtained from the Hanford 
Site in the State of Washington, which is 
one of the candidate locations for a 
nuclear waste repository. The compres- 
sive strength of the basalt samples used 
for testing ranged from 25,000 to 50,000 
psi. 

Table 1 summarizes the test results ob- 
tained in the Colorado Red Granite with 
and without jet-assisted mechanical cut- 
ting. As shown in the table, test param- 
eters involved various combinations of 
jet pressures and cutter penetration 
depths. For all tests a 0.025-in jet was 
used at a 2-in standoff and 3-in cut 
spacing. The cutter force measurements 



contained in table 1 are plotted in fig- 
ures 1 through 3. Included in these fig- 
ures are the normal and rolling forces 
experienced by the cutter as a function 
of jet pressure for the three depths of 
cutter penetration (0.10, 0.20, and 0.30 
in). As regards the normal force, the 
water-jet assist is seen to produce a 
maximum reduction of about 25 pet within 
the levels of parameters tested. The 
rolling force is influenced to a higher 
degree by jet application with an average 
35-pct reduction compared to mechanical 
cutting only. The results do not, of 
course, reflect the optimum cutting con- 
ditions, meaning that additional reduc- 
tions in cutter normal and rolling forces 
appear feasible through optimization of 
all pertinent system parameters. In con- 
trast to the normal and rolling forces, 
water-jet assist does not seem to affect 
the side force acting on the cutter. The 
side force data listed in table 1 exhibit 
large scatter and therefore do not allow 
derivation of any firm conclusions re- 
garding this 'effect. 



TABLE 1. - Results of water-jet-assisted disk cutting tests 
with jet preceding cutter in groove 

(Colorado Red Granite, 15.5 in - 90° disk cutter, 0.025-in 
jet, 3.0-in cut spacing, 2-in jet standoff) 



Cutter penetration 



Jet 

pressure. 

psi 



Average cutter forces, lb 



Side Normal Rolling 



0.10 in. 



0.20 in. 



0.30 in. 



Dry 1 

5,000 

10,000 

15,000 

20,000 

Dry' 

5,000 

10,000 

15,000 

20,000 

Dry 1 

5,000 

10,000 

15,000 

20,000 



1,569 
1,843 
1,610 
1,452 
1,843 

4,010 
4,323 
3,607 
4,984 
4,278 

5,177 
7,577 
4,143 
3,069 
4,251 



21,343 
17,066 
16,651 
17,459 
21,563 

32,441 
29,525 
26,273 
23,246 
28,266 

49,674 
42,027 
40,753 
38,344 
38,763 



1,366 
702 
864 
742 
963 

2,320 
2,086 
1,916 
1,581 
1,809 

5,266 
4,442 
4,030 
3,721 
3,703 



'Mechanical cutting without jet assist. 



66 




JET PRESSURE, 10° psi 

FIGURE 1. - Colorado Red Granite, force versus 
jet pressure, 0.10-in cutter penetration. 

A series of linear cutting tests with 
jet assist was also run in Hanford 
basalts. This particular test program is 
still continuing; thus only test results 
generated to date are presented herein. 
Figure 4 displays the normal and rolling 
force measurements for various jet pres- 
sures and two standoff distances at a cut 
spacing and penetration of 2.5 and 0.15 
in, respectively. Jet size was 0.02 in. 
Again, the cutter forces are reduced due 






1 


1 1 1 


B 


M 
O 








^ 




— Mechanical cutting 




UJ 








u 








cc 


• 






£20 




• 

• 




CO 








z 












• 




_1 








_l 








o 








or 
in 


1 


1 1 1 





5 10 15 20 25 

JET PRESSURE, I0 3 psi 

FIGURE 2. - Colorado Red Granite, force versus 
jet pressure, 0.20-in cutter penetration. 

to application of water jets with higher 
pressures and smaller standoff distances 
producing greater force reductions. 
Again, the water-jet assist influences 
the rolling force to a higher degree than 
it does the normal force. This finding 
is of major importance since most TBM's 
become torque limited before their thrust 
capacity is ever reached. As the rolling 
force on the cutters is the parameter 
that determines the cutting torque on a 
TBM, the fact that major reductions can 
be achieved in the rolling force compo- 
nent through the application of low-pres- 
sure water jets emerges as a significant 
benefit. 



MECHANISM OF WATER- JET-ASSISTED DISC CUTTING 



As discussed earlier, the initial ef- 
forts toward development of a water- 
jet-assisted cutting system for applica- 
tion to hard-rock TBM's were directed at 



using high-pressure jets to create kerfs 
in virgin rock to promote chip formation. 
The mechanism involved in this applica- 
tion was a simple slotting action that 



67 



60 



ro 
O 



45 - 



L±J 

o 
or 
p 



30 



o 





1 


1 1 


A 






^-Mechanical cutting 




- 


• 


• 

• 


< 




1 


1 1 






JET PRESSURE, 10^ psi 

FIGURE 3. - Colorado Red Granite, force versus 
jet pressure, 0.30-in cutter penetration. 



KEY 
CZI Normal force 
|( S j £23 Rolling force 
2 ksi 




30 



20 



— 10 



Water jet, 
2- in standoff 



FIGURE 4. - Normal force measurements in 
Umatilla Basalt with and without jet assist. 



made use of the high, concentrated power 
which water jets are capable of deliver- 
ing. The mechanism by which a low- 
pressure jet assists the disk cutting 
performance is quite different, probably 
encompassing several factors. It is be- 
lieved, however, that three factors play 
a major role in this respect. The first, 
and perhaps the most important, mechanism 
is the removal of the crushed zone imme- 
diately surrounding the cutter groove. 
Visual observations of cutter paths to- 
gether with photomicrographs of the 
crushed and fractured zone beneath the 
cutter have confirmed the occurrence of 
such a mechanism. The second mechanism, 
which is believed to take place with jet 
assist, is the so-called hydrof racturing, 
whereby the water under pressure pene- 
trates and enlarges the subsurface cracks 
produced due to the mechanical cutting 
action. Some of these cracks are en- 
larged and extended to a length suffi- 
cient to create additional chip forma- 
tion, increasing the total yield of rock 
material removed by the system. Lastly, 
the water jet appears to lubricate the 
cutting path and provide a cooling effect 
on the cutter. This mechanism coupled 
with the removal of the crushed zone is 
believed to contribute to improved cutter 
ring life. 

As previously stated, the principal at- 
traction of low-pressure water jets is 
their low power requirements. For exam- 
ple, a 0.020-in-diam jet operating at a 
pressure of 5,000 psi generates a flow 
volume rate of about 0.70 gal/min and re- 
quires approximately 2.5 hp at the pump. 
In addition to these advantages compared 
to high-pressure jets, use of low pres- 
sure also improves equipment reliability 
and practically eliminates most of the 
safety concerns that are raised when con- 
sidering the application of high-pressure 
jets on TBM's. 

It is not necessarily a requirement or 
a feasible approach that every cutter on 
a TBM should be fitted with a water jet 
in order to achieve the desired level of 
improvements in excavation rates and cut- 
ter lives. A number of jets strategical- 
ly placed on the cutterhead in areas 
where the cutting action is most diffi- 
cult, i.e., the gage and center cutters, 



68 



may serve the purpose and provide the de- 
gree of performance enhancement that is 
necessary to warrant the additional cost 
of incorporating water jets on TBM's. 
This goal can be accomplished by carry- 
ing out full-scale field demonstration 
tests with a water-jet-assisted TBM for 
determining the optimum locations of jet 



nozzles on the cutterhead. Such a test 
program can also furnish data relating to 
the impact of jet assist on cutter wear. 
It is very difficult to investigate the 
wear effects in the laboratory since the 
amount of rock cut generally is not suf- 
ficient to produce any measurable degree 
of wear on the cutter. 



SUMMARY 



The laboratory test results obtained 
thus far indicate that low-pressure water 
jets directed in the disk cutter path 
offer a significant potential for en- 
hanced TBM performance. A side benefit 
appears to be extended cutter life, mean- 
ing lower excavation costs. Because of 
low pressure requirements, it is felt 
that a water-jet-assisted cutting system 
can be installed on a TBM without any ma- 
jor difficulty. The pumping equipment 



required to generate these low pressures 
is commercially available with proven re- 
liability for operation over long periods 
of time . 

Further testing is planned to improve 
system effectiveness and to achieve the 
optimal configuration of jet-assisted 
disk cutting. Greater benefits of jet 
assist than those reported in this paper 
appear feasible through additional opti- 
mization studies. 



69 



INVESTIGATION OF OPTIMIZING TRAVERSE SPEED OF WATER- JET-ASSISTED DRAG PICKS 
By R. J. Evans, 1 H. J. Handewith, 2 and C. D. Taylor 3 



ABSTRACT 



A cutterhead was designed and fabri- 
cated at the Bureau of Mines Pittsburgh 
Research Center to evaluate the rock- 
weakening effect, when using water- 
jet-assisted cutting, that may result 
from increased travel speed using 
moderate-pressure jets (3,000 to 9,000 
psi). Two pick types, conical and flat, 
were analyzed for speeds ranging from 30 
to 300 ft/min. The objective was to es- 
tablish traverse speed threshold levels 



that are necessary to maintain the opti- 
mum reduction in cutting tool forces. In 
addition, measurements were taken to mea- 
sure dust reduction when using water-jet 
assist. Results indicate that when em- 
ploying water-jet-assisted cutting, only 
a negligible reduction in the rock- 
weakening effect was observed at higher 
speeds. As an added benefit, significant 
dust reduction was evident when employing 
water-jet-assisted cutting. 



INTRODUCTION 



The Bureau has initiated a research 
program in water-jet-assisted cutting to 
determine the improvements possible in 
cutting performance and health and safe- 
ty. Underground coal mining machines, 
such as the longwall shearer and the con- 
tinuous miner, cut with bit speeds rang- 
ing from 400 to 800 ft/min. To measure 
water-jet-assisted cutting effectiveness 
at higher speeds, a full-face cutterhead 
was designed and fabricated at the 
Pittsburgh Research Center. It contains 



six jet-assisted drag bits as shown in 
figure 1. The cutterhead has a 36-in 
diam and is outfitted to use either flat 
or conical bits. The bit spacing, shown 
in figure 2, is 2-1/2 in between each bit 
and double tracking at the gage bits. 
The cutterhead was designed to bring 
high-pressure water through a swivel 
mounted on the rear of the cutterhead. 
The water is then transported through 
holes drilled in the body of the cutter- 
head to the six individual bits. 



APPROACH 



The traverse speed cutterhead was 
mounted on a raise boring machine gearbox 
(fig. 3) which was driven by a fixed- 
displacement, variable-stroke hydraulic 
pump. The cutterhead was flange-mounted 
on a 5-ft section of Drilco 11-in-diam 
drill pipe with DI-22 threaded connec- 
tions. Rotation was varied from 1/2 to 
40 r/min. The maximum available thrust 
was 120,000 lbf. Torque was a demand 

Supervisory civil engineer, Pittsburgh 
Research Center, Bureau of Mines, Pitts- 
burgh, PA. 

^Project research supervisor, Boeing 
Services International, Inc. , Pittsburgh, 
PA. 

^Industrial hygienist, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, PA. 



function of thrust, which generated a 
maximum hydraulic pressure of 5,000 psig 
at full pump stroke. The water-jet sys- 
tem was powered by two 10-gal/min posi- 
tive displacement triplex pumps generat- 
ing a maximum pressure of 9,500 psi. 
Flow (pressure) control was achieved by 
varying the running speed of the diesel- 
engine-powered pump. The water jet was 
mounted in front of the bit and directed 
to impinge less than 5 mm in front of the 

bit cutting tip, as shown in figure 4. 
The water for the jet-assist cutting pro- 
cess was filtered through a 100-um fil- 
ter, delivered through the back of the 
drive train gearbox, and transported 
through the drill pipe and then through 
the high-pressure swivel, which was lo- 
cated in the back of the cutterhead. 



70 




Gauge bits 
double 
tracking- 



mammmsss 

FIGURE 1. - Water-jet-assisted full-face 
cutterhead. 




FIGURE 2. - Cutterhead bit spacing (dimen- 
sions in inches). 




FIGURE 3. - Traverse speed cutterhead. 



71 




FIGURE 4. - Water-jet-assisted cutting bit 
and nozzle. 

The test material was a German sand- 
stone with a compressive strength of 
19,000 psi, a bulk density of 156.6 lb/ 
ft 3 , and 50- to 55-pct quartz content. 
Two 4- by 4- by 2-ft-wide blocks of this 
material were mounted on the test stand 
and cemented together with a 3- to 4-in 
layer of concrete around the outside, as 
shown in figure 5. A 13-in-diam pilot 
hole was drilled through the block to 
guide the cutterhead as it cut through 
the block. The cutterhead was equipped 
with a pilot fixture that fit into the 
pilot hole as shown in figure 6. 

Dust levels were measured while cutting 
dry and with water-jet assist by drawing 
a portion of the dust generated from the 
cutting surface into a dust box. While 
in the box, respirable dust was separated 
from nonrespirable dust using two 10-mm 
nylon cyclones and sampling heads. Two 
RAM-1 real-time dust sampling instru- 
ments were used to draw respirable dust 
from the cyclone sampling heads and to 



Block I 2 ft 
Block 2 2 ft 




Typical concrete 
case, 3 to 4 in- 
wide 



FIGURE 5. - German sandstone test block, uncon- 
fined compressive strengths: Sandstone, 19,000 
psi; concrete, 6,000 to 7,000 psi (est.). 

Rotation 




FIGURE 6. - Sandstone block and traverse speed 
cutter test setup. 

continuously measure the dust concentra- 
tions. The concentration data from each 
instrument were recorded by a dual-pen 
strip-chart recorder. The areas under 
the recorded curves were compared to de- 
termine the relative quantities of dust 
generated when operating with and without 
water-jet assist. 



RESULTS 



Today, nearly all laboratory and some 
field tests are conducted at a constant 
depth of cut (DOC) with variable force 
rates. However, in actual practice, all 
mechanical mining machines operate with 



fixed force rates and variable force or 
penetration. The traverse rate tests 
conducted here were closer to actual 
practice than to laboratory testing. 



72 



The most significant observation was 
that the German sandstone could not be 
cut without water-jet assist. Figure 7 
shows the results when cutting dry for 
less than 1 minute. The conical bit was 
worn to the point of being unrecogniza- 
ble. When cutting dry, grinding steel 
rather than cutting rock was apparent. 
However, when using 6,000-psi jet assist, 
large chips were cut as shown in figure 8 
with much less bit wear. Over the tra- 
verse speed range of 35 to 300 ft/min, no 
change in water-jet-assisted cutting ef- 
ficiency was noted. 

To measure efficiency, the use of en- 
ergy volume (EV) as a measure of rock 
boring rate was proposed by Ross and 
Hustrulid 4 in 1972. This EV value was 
used in this study to evaluate the rock- 
weakening effect. 



EV = 



(2, x N x Ib) + (Tx|) 
0.6^ D 2 x AR 



where 



AR = advance rate of cutter into 
the rock sample, ft/h, 

N = revolution rate of cutter- 
head, r/min, 

Energy 



and 



EV = tt^ — = energy per unit 
Volume 1*4 

volume of in situ 

fractured rock, 

(in«lbf)/in 3 , 

To = cutterhead drive torque, 
fflbf , 

T = system thrust on cutterhead, 
lbf, 

D = diameter of cutterhead or 
test bore, ft. 



4 Ross, N., and W. Hustrulid. Develop- 
ment of a Tunnel Boreability Index (U.S. 
Bur. Reclamation contract 1 4-06-D-6849, 
Dep. Mining, CO Sch. Mines). Feb. 1972, 
351 pp.; NTIS REC-ERC-72-7. 





FIGURE 7. - Dry cutting of a 19,000-psi sand- 
stone with a conical pick. 

Without using water-jet assist, the en- 
ergy volume (EV) rate was greater than 
200,000 (in«lbf)/in 3 with an instantane- 
ous advance rate of less than 0.75 ft/h 
as shown in figures 9 and 10, respective- 
ly. However, when employing water-jet 
assist at 6,000 psi, the energy volume 
rate was less than 35,000 (in«lbf)/in 3 
with corresponding advance rates greater 
than 3 ft/h (figs. 9-10). The fastest 
advance rate, 88.5 ft/h, was achieved 
when using water-jet-assisted cutting at 
6,000 psi. The EV was 1,150 (figs. 9- 
10) , and the traverse rate was 40 ft/min. 

There was a noticeable difference in 
efficiency when nozzles became clogged. 
One test had a partially plugged jet noz- 
zle operating at 0.26 gal/min which re- 
sulted in an EV of 77,600 (in«lbf)/in 3 
(fig. 9). This indicated that nozzle 
flow rates of at least 1 gal/min must be 
sustained for optimum cutting of German 
sandstone. 



73 




FIGURE 8. - Rock chips when cutting with water-jet-assist. 



Three tests with an advance rate in ex- 
cess of 20 ft/h had a large departure 
from the rest of the data, which were all 
less than 14 ft/h as shown in figure 10. 
The reasons for these higher advance 
rates are not obvious. As shown in fig- 
ure 11, advance rate is plotted as a 
function of rotation rate. With a fixed 
thrust force, the advance per revolution 
steadily decreased nonlinearly as the 
rotation rate increased. Industry has 
always believed there is a linear rela- 
tionship between rotation rate and ad- 
vance rate. It has been demonstrated 
that advance rate can be increased by 
deeper DOC (in/r) or by increasing the 
rotation rate. However, some other phe- 
nomenon caused advance rates in excess of 
20 ft/h, as shown in figure 10. Too few 
data exist to explain this phenomenon. 



In the paper by Ross and Hustrulid, 5 
for efficient cutting it was proposed 
that the average EV, expressed as (in 
•lbf)/in 3 , would approximate the average 
unconfined compressive strength of rock, 
expressed as lbf/in 2 ). The quantity was 
proposed as a measure of cutting effi- 
ciency. Since dry cutting was obviously 
not efficient, only wet cuts were evalu- 
ated as shown in table 1 to identify the 
average EV for all jet tests using both 
conical and flat bits. Although the flat 
bit was 19 pet less efficient than the 
conical bit, the flat bit produced the 
highest net advance rate. The reason for 
this is beyond the scope of this report 
and warrants more study in the future. 
Average EV of all water-jet-assisted 

^Work cited in footnote 4. 



74 



300 



250 - 



•o 200 h 
o 



UJ 

_i 
o 

> 

rr 
u 

Q- 

>- 

cc 

UJ 

z 

UJ 



150 - 



100 



50 



1 1 


1 1 

D 


1 




A 




- 


KEY 
■ Conical bit 
a Flat bit 

□ Conical bit, dry cutting 
a Flat bit, dry cutting 


" 0.26 gal/mir 
per nozzle — 






Greater than 
- per nozzle-, 


1 gal/min 
\ 


_ 




H 


n 



1.6 

1.4 
1.2 
1.0 



ui 
o 

z 

o 
< 



KEY 

■ Conical bit 

a Flat bit 

a Conical bit, dry cutting 

a Flat bit, dry cutting 



Expected range 




8 



^L 



_L 



Unexpected 
range 



4 8 12 16 20 24 28 

ADVANCE RATE, ft/h 

FIGURE 10. - Advance versus advance rate. 



1.4 


i 


1 1 1 1 l 

KEY 




1.2 


\ ■, 


|n/r = 0.64+ (-0.18) in* rpm 




\ 
\ 


R 2 = 0.71, conical bit 




1.0 


_ \ A *. 
\ 


In/r= 1.24+ (-0.42) in* rpm 

R2 = 0.62, flat bit 


- 


.8 


\ 
\ 
\ 
\ 
\ 


* Natural logarithm 


- 


.6 
4 


V \ 

~ \ ^ 
\ \ 

\ \ 


- 


.2 


AA 

1 


i i""^ i * 





100 200 300 

TRAVERSE RATE, ft/min 

FIGURE 9. - Cutting of 19,000-psig quartz 
sandstone. 

testing was 19,490 (in«lbf )/in 3 . The re- 
ported average unconfined compressive 
strength for the German sandstone was 
19,000 psi (at CSM by Ropchan), 6 sug- 
gesting a reasonable confirmation for 
the Ross and Hustrulid hypothesis that 
EV approximates unconfined compressive 
strength for efficient cutting. 

6 Ropchan, D., F. D. Wang, and H. J. 
Wolgamott. Application of Water- Jet- 
Assisted Drag Bit and Pick Cutter for the 
Cutting of Coal Measure Rocks (U.S. Dep. 
Energy contract ET-77-a-01-9082, CO Sch. 
Mines). Apr. 1980, 120 pp.; NTIS DOE/ 
ET/1 2463-1. 



TABLE 1. - Energy volume relations with water-jet assist 

Conical bit, model 070, average of 4 tests (in'lbf )/in 3 . , 

Flat bit, model K-107, average of 10 tests (in-lbf )/in 3 . . 

Average of all 14 tests (in*lbf )/in 3 . . 

Reported average unconfined compressive strength of German 
sandstone lbf /in 2 . . 







5 10 15 20 25 30 35 

CUTTERHEAD ROTATION RATE, rpm 
FIGURE 11. - Advance cutterhead rotation rate. 



16,768 
20,579 
19,490 

19,000 



75 



To further confirm the test results, 
scalar and force relations, and calcula- 
tion assumptions, a comparison was made 
with previous testing conducted in the 
German sandstone in 1980. 7 This compari- 
son is shown in table 2. Although there 
were slight differences in water pressure 
and DOC, normal, cutting, and resultant 
forces were in good agreement for the 
conical bit. Ropchan did not test the 
flat bit. This confirms that scalar re- 
lationships can be established for the 
forces reacting on a single cutting tool 
and on an interactive array of such cut- 
ting tools. A significant departure from 
the Ropchan tests was the spacing between 
interactive cutting tools. Ropchan used 
a tool spacing of 1.5 in, whereas the 
spacing used here was 2.5 in. With near- 
ly the same cutting tool force reactions, 
the tests conducted here produced 167 pet 
more in situ rock volume at the same bit 
force or EV rate. 

TABLE 2. - Cutting of German (19,000-psi) 
sandstone with conical bits 





Force, lbf 


Test 


Nor- 
mal 


Cut- 
ting 


Result- 
ant 


Single-tool, by CSM: l 

5,000-psig water 
iet 


3,860 

2,042 

3,850 


3,501 
2,340 

2,340 


5,220 
3,121 


Multitool, by Bu- 
Mines: 2 6,000-psig 


4,500 



Colorado School of Mines; depth of cut 
was 0.5 in at a 1.5-in tool spacing. 

2 Calculated from gross system forces 
reacting on an array of 6 cutting tools; 
depth of cut was 0.454 in at 2.5-in tool 
spacing. 

During the water-jet-assisted tests, 
data describing several operation func- 
tions were simultaneously monitored and 
recorded. The dependent or controlled 
function were — 

FN(KLBF) = Normal or thrust force in 
thousands of pounds, 



'Work cited in footnote 6. 



RPMs = Cutterhead revolutions per 
minute , 

PW = Water pressure, psig. 

Independent functions were — 

INCHES = Advance in inches of the 
cutterhead into the test 
rock over the test period 
in seconds, 

FT(PSIG) = Torque force measured as a 
function of the hydraulic 
gauge pressure, psig, 

RF = Volume rate of water jet 
flow, psig. 

One conclusion reached in this study is 
that there is a minimum threshold of 
thrust below which the German sandstone 
cannot be bored. This threshold was not 
reached for dry cutting and was obviously 
much higher than that required using 
water-jet assist. 

Tool wear, after advancing less than 
1 in into the test block, was excessive, 
as indicated by the conical bit shown in 
figure 8. All six cutting tools failed 
in this dry test. When water-jet assist 
was used, only occasional tool failures 
occurred. 

Dust sampling results shown in figure 
12 indicate that dust concentrations were 



H 

o 

Z> 
Q 
Ld 

cr 

H 

z> 
a 



w 


c^^ 


1 

• 


• 


1 




80 


- 








-II 


60 








• 




40 

on 


' 


1 




1 


-i 







30 



10 20 

CUTTING ROTATION RATE, rpm 

FIGURE 12. - Dust reduction using water-jet- 
assistwith flat bits (compared with dry cutting) 
as a function of cutterhead rotation rate. 



76 



reduced 60 to 85 pet when using water-jet 
assist versus operating dry. The per- 
centage reductions for these tests (con- 
ducted with 6,000-psi water pressure, 



30,000-psi thrust, and flat bits) in- 
creased as the cutting head rotation 
speed was reduced. 



CONCLUSIONS 



The German sandstone was far more dif- 
ficult to cut than originally expected. 
Dry cutting was impossible. Water-jet- 
assisted cutting with 6,000 psi resulted 
in dramatic improvement. No measurable 
degradation in rock-weakening effect was 
found over the tested traverse rate range 
of 35 to 300 ft/min in German sandstone. 

The following are additional pertinent 
results: 

1. With water-jet-assisted cutting, 
the EV was nearly equal in value to the 
unconfined compressive strength of rock, 
indicating that cutting was in the effi- 
cient EV range. 

2. Water-jet assist was obviously ef- 
fective in reducing bit wear and cutting 
tool forces. 

3. EV level for all dry cutting was in 
excess of 200,000 (in'lbf )/in 3 . 



4. EV levels averaged 19,490 (in«lbf)/ 
in 3 , using 6,000-psig water-jet assist at 
flow rates in excess of 1 gal/min and 
0.6-mm jet nozzles. 

5. Force reactions measured on a sin- 
gle jet-assisted cutting tool in German 
sandstone by Ropchan compared favorably 
with those forces calculated on the in- 
teractive array of cutting bits tested by 
the Bureau of Mines and Boeing Services 
International. 

6. Scalar force-rate relationships can 
be established between a single cutting 
tool and an interactive array of cutting 
tools. 

7. The maximum instantaneous advance 
rate recorded on one test was 88.5 ft/h. 



77 



OPTIMIZATION OF WATER- JET SYSTEMS FOR MINING APPLICATIONS 
By Dr. James M. Reichman 1 



INTRODUCTION 



Mining depends upon the ability to ef- 
ficiently cut and drill rock so that it 
can be economically excavated and pro- 
cessed. Ideally, the most efficient min- 
ing operation would be continuous. Thus, 
much is contingent upon the longevity and 
reliability of the equipment used. The 
need to extend machine life is emphasized 
by the negative effects of cutter and bit 
wear and, indirectly, decreased equipment 
reliability due to the higher loads re- 
quired at wear points. In the past, at- 
tempts to improve machine life have fo- 
cused on the "hardening" of working 
parts. However, metallurgy and engineer- 
ing are rapidly approaching the limits of 
present technology. 

An alternative to "hardening" of mining 
equipment is to develop a cutting tech- 
nique that minimizes abrasive wear, re- 
duces the required mechanical cutting 
force, or decreases the amount of rock 
cutting or drilling needed to effect 
breakage. The use of water jets with 
mining equipment is one method of achiev- 
ing these goals. The optimization of 
this approach is the subject of this 
paper. 

Water jets can be adapted to mining ap- 
plications in various ways, including 
pure water-jet cutting, water-jet cutting 
with mechanical assist, mechanical cut- 
ting with water-jet assist, and water-jet 
cutting with abrasives. The pertinent 
parameters involved in pure water-jet 
cutting are shown in figure 1. Water-jet 
cutting produces narrow kerfs in the 
rock. To remove all the rock, kerfs must 
be made close together; as figure 2 indi- 
cates, rock type dictates kerf spacing. 



Rock type also determine kerf depth and 
cutting effectiveness. 

Water-jet cutting with mechanical as- 
sist uses water jets to cut kerfs at a 
fixed spacing and then employs mechanical 
force to break the rock between the 
kerfs. Uncut ridges may be broken using 
a drag-type cutter. In this method the 
water jet accomplishes the major cutting 
task, while the mechanical cutting is 
secondary. 

In techniques that employ mechanical 
cutting with water-jet assist, the me- 
chanical cutting force is generally suf- 
ficient to cut the rock. The water jets 
are used to reduce the required cutting 
force, either by weakening the rock or by 
improving the cutting conditions. As 
shown in figure 3, the water jet can 
weaken the rock so that it fails in ten- 
sion. The water jet can also be used to 
clear the cuttings away from the bit, 
permitting the machine to apply high 
stress directly to the rock. 

The use of a water jet with an en- 
trained abrasive adds a new dimension to 
water- jet cutting. The introduction of a 
high-velocity abrasive particle (fig. 4) 
extends the cutting depth by a factor of 
5 or more over a pure water jet. In ad- 
dition, the sensitivity of the water jet 
to various rock types is reduced. 

The following sections discuss some ap- 
plications of the various water-jet tech- 
niques . To achieve the performance goals 
and the most economical system, it is im- 
portant to optimize the use of the water 
jet for each application. This optimiza- 
tion procedure will also be discussed. 



WATER- JET APPLICATIONS 



PURE WATER JETS 



technique energy intensive. 



In many 



The high specific energy required 
to excavate large amounts of materi- 
using pure water jets 



al 



makes this 



cases the inhomogeneity of the material 

1 Director, High Pressure Technology, 
Flow Industries, Inc., Kent, WA. 



78 



Nozzle 




25 



KEY 

t Standoff distance Vf Traverse velocity 
do Nozzle diameter w Width of cut 
P Nozzle pressure h Depth of cut 

FIGURE 1. - Water-jet cutting parameters. 





Side jets cutting 
ahead of pick 



Jet hitting pick 
at contact point 



FIGURE 3. 
assist. 



Mechanical cutting with water-jet 



makes hydraulic mining impractical. Min- 
ing with pure water jets makes economic 
sense only under special conditions, such 
as where the seam is on an angle and the 
material is friable (as in coal mining). 
It is, therefore, advantageous to iden- 
tify applications where minimal material 
removal produces the desired results. 

An example of "minimal removal" is 
disking and/or slotting of a drilled hole 
(fig. 5). In this case the water jet is 



O 

1 

IT) 



3 

en 



rr 

1x1 

h- 

UJ 

< 

o 

UJ 
_1 
N 
N 
O 



20 



15 



10 



I ' 1 

1 KEY ' 

a Charcoal 

• Granite 

t Wilkeson Sandstone, 0.014-indiam. 

■ Wilkeson Sandstone, 0.018-in diam 



_l 



I 



'0246 
CUTTING VELOCITY, in/s 

FIGURE 2. - Kerf spacing. 



High-pressure water 







Coherent high-velocity jet 



ti Abrasives 



— Patented nozzle 
configuration 



— Focused particle 
jet 



■Target 



FIGURE 4. - Abrasive water-jet schematic. 

used to create slots along the side of 
the hole. These slots allow the direc- 
tion of fracture to be determined before 
blasting, enabling more efficient break- 
age of the rock. When the back of a hole 
is disked, "boot leg" is eliminated and 
more of the rock in a face is recovered. 



79 




Slot fracturing 




Disk fracturing 
FIGURE 5. - Water-jet slotting and disking. 

In this example, the ability of a water 
jet to cut a rock without mechanical con- 
tact is more important than the energy 
required to do the cutting. This is be- 
cause the minimal cut has a pronounced 
effect on the overall mining process. 
This method can be used to the best ad- 
vantage by optimizing the configuration 
of the blast holes to take full advantage 
of the improved breakage. 

WATER- JET CUTTING WITH 
MECHANICAL ASSIST 

In this method, the water jet provides 
the principal means of cutting while the 
mechanical assist is used to break the 
rock left uncut by the jets. The thrust 
and torque of such a cutting device are 
much lower than for a similar mechanical 
cutter, but the power requirements are 
higher in many cases. Hence, the most 
appropriate applications are when thrust 



and torque reduction are of prime 
importance. 

An example of this cutting method is 
a high-pressure water-jet drill (fig. 6). 
In this type of drill, the cutting wa- 
ter jets leave ridges, which are then 
broken off as the drill progresses. The 
drill requires thrusts of several hundred 
pounds; the reduced cutter loads extend 
operating life. This type of drill is 
especially useful in abrasive rock, when 
equipment must be light and maneuverable , 
when hand operation is required, and when 
a longer-than-seam-height drill is re- 
quired. This latter application is one 
where low torque and thrust are critical, 
due to transmission around a curve. 

Although this type of drill has many 
inherent advantages, the energy consump- 
tion and capital cost could be prohibi- 
tive. It is, therefore, critical to op- 
timize the cutting process for a particu- 
lar application. 

MECHANICAL CUTTING WITH WATER- JET ASSIST 

Water jets can be used to augment the- 
cutting process of a mechanical cutter. 
This reduces the required cutting force 
and allows pick or cutter spacing to be 
increased, improving overall cutting per- 
formance. In addition, water jets im- 
prove the ability of a machine to effi- 
ciently cut harder rock. This method of 
cutting with water jets is particularly 
useful when machine size is not the crit- 
ical factor, since most of the cutting is 
accomplished mechanically. 

Large mining equipment for such appli- 
cations includes longwall shearers, con- 
tinuous miners , road headers , and raise 
and blind shaft borers. In this applica- 
tion the jet is used to directly assist 
the cutter (fig. 7). In underground 
tests, this equipment improved cutting, 
reduced dust and wear , and eliminated 
sparking. 

To optimize this type of cutting sys- 
tem, it is necessary to determine the 
best method to directly assist only those 
cutters in contact with the rock. This 
is especially important in machines that 
employ a rotating drum, only a portion of 
which is actually in contact with the 
cutting face. 



80 




FIGURE 6. - Water-jet drill. 



ABRASIVE WATER- JET CUTTING 

The use of a water jet with an en- 
trained abrasive allows a deep cut (up to 
24 in) to be made in rock. The resulting 
slot can be used to isolate a portion of 
rock from the main body, enabling selec- 
tive excavation. 



Some applications of this technique are 
improved perimeter control, as shown in 
figure 8, selective mining, and roof and 
floor control. Optimization involves ad- 
justing the abrasive water jet itself for 
the particular process, as well as inte- 
gration of the technique with the entire 
mining or tunneling process. 



WATER-JET OPTIMIZATION 



All systems utilizing water jets must 
be optimized. This often means simply 
identifying the pressure and flow rate 
that most efficiently produce the desired 
result. In other cases, optimization in- 
volves reducing energy waste. The fol- 
lowing subsections describe optimizations 
for some selective applications. 

WATER- JET DRILL WITH MECHANICAL ASSIST 

When using water jets as the primary 
cutting tool, the objective is to develop 



a drill with minimum mechanical assist. 
The drill should be capable of drilling 
effectively in all rocks. However, as a 
general rule for optimum performance, the 
harder the rock the higher the operating 
pressure. 

To develop the drill, an acceptable 
horsepower level and a maximum allowable 
flow rate must be established. It is im- 
portant to determine these parameters at 
the outset, since the available power de- 
termines the initial and operating cost 
of the equipment. Also, excessive water 



81 



Water-jet 
nozzles 




Water -jet 
nozzles 




FIGURE 7. - Water-jet shearer and roadheader. 



82 




FIGURE 8. - Perimeter control with abrasive jet. 



requirements can result in supply and 
disposal problems , as well as floor con- 
trol problems. By setting these para- 
meters, the range of available pressures 
can be determined. The actual pressure 
at which the drill is operated should 
be the minimum pressure required to 
obtain an acceptable cutting speed in 
a particular application. Minimizing the 
pressure increases the reliability of 
high-pressure components. 

Optimizing drill performance only 
starts with establishing the operating 
pressure; the remainder of the process 
involves determining the most critical 
characteristics of the drill. This re- 
sults from contradictory trends in the 
drill performance. For example, if the 
water jet is used to cut grooves which 
are then broken off by a mechanical cut- 
ter, groove depth must be maximized. As 
shown in figure 9, there is an inverse 
relationship between the depth of cut and 
the force needed to break the uncut 
ridge. However, it is not always possi- 
ble to cut a deep kerf using available 
pumps. As a result, it may be necessary 
to use higher than desired drill thrust 
to achieve acceptable cutting. 

A second important compromise in the 
drill design is the effect of increasing 
the thrust on the drill. The positive 
result is that the drill rate increases 
dramatically with thrust (fig. 10A) . 
However, this advantage is countered 
by increased drill wear (fig. 10B). The 
wear rate increase can have important 



O 

rr 
o 

u_ 

o 

z 

h- 

H 

o 



Fixed jet spacing 



KERF DEPTH 

FIGURE 9. - Force versus groove depth. 



< 



Q 



a: 
< 

LlI 



THRUST THRUST 

FIGURE 10. - Effect of thrust on drilling rate 
and wear (constant-diameter hole). 

ramifications on the economics of the 
drilling process that offset the in- 
creased speed. 

The optimization process can result in 
a drill that is close to being as energy 
efficient as a conventional drill, per- 
forming at comparable or better drilling 
rates while reducing bit wear by a factor 
of 4 or 5. Drills of this type have al- 
ready been developed for roof bolting ap- 
plication, and hard-rock drills are cur- 
rently being examined. 

WATER- JET-ASSISTED MECHANICAL CUTTING 

The application of water-jet-assisted 
mechanical cutting is widely being exam- 
ined for improved performance of min- 
ing and tunneling machines. Two methods 
of applying the jet assist have been 
examined: direct assist on the bit and 



83 



slotting between cutters. 2 in both meth- 
ods the performance is optimized by main- 
taining a high power density in the jet 
at the rock. Retaining high power densi- 
ty on typical mining machines is not al- 
ways easy. 

Typical mining machines using either 
picks or disk cutters do not allow the 
jet to be close to the cutting surface. 
This has an adverse effect on the cutting 
effectiveness of the jet, as illustrated 
in figure 11. There is a dramatic de- 
crease in depth with distance which ad- 
versely affects performance. This can be 
compensated for by increasing the jet 
pressure or diameter. Increasing either 
of these increases the power required for 
the machine. An alternate approach is to 
use small concentrations of polymer in 
the jet. This has been demonstrated 3 to 
increase jet coherence, and thus cutting 
effect with distance is improved. This 
could prove to be an efficient alterna- 
tive to increasing the jet power and 
still achieving the desired decrease in 
cutting force. 

In many mining machines only a portion 
of the cutters are actually in contact 
with the rock. Only a third of the cut- 
ting bits shown on the shearer drum (fig. 
12) are cutting at any one time; during 
about two-thirds of the drum rotation the 
cutters are in the air. If all the cut- 
ters are continuously assisted with water 
jets, two-thirds of the energy is wasted. 
This problem can be eliminated by acti- 
vating the jets as they come in contact 
with the rock. This can be achieved by 

o — 

"•Hennecke, J., and L. Baumann. Jet As- 
sisted Tunnel Boring in Coal-Measure 
Strata. Paper in Fourth International 
Symposium on Jet Cutting Technology 
(Univ. Kent, Apr. 12-14, 1978). BHRA 
Fluid Engineering, Cranfield, Bedford, 
England, 1978, pp. J1 ff. 

3 Franz, N. C. Fluid Additives for Im- 
proving High Velocity Jet Cutting. Paper 
in First International Symposium on Jet 
Cutting Technology (Univ. Warwich, Apr. 
5-7, 1972). BHRA Fluid Engineering, 
Cranfield, Bedford, England, 1972, pp. A7 
ff. 



35 
30 
25 

20 



Pressure at orifice = 5,000 psi 
Orifice diam = 10 mm 
Traverse velocity - 61 mm/s 
3-D nozzle 



20 



100 



40 60 80 

STANDOFF DISTANCE, mm 

FIGURE 11. - Water-jet cutting effectiveness 
with standoff. 



120 




FIGURE 12. - Cutting drum contact area with rock. 

segmented valves , which supply water 
to the segment of the head that is in 
contact with the rock. Examples of each 
are discussed below. 

Flow Industries has 
quencing system for a 
header. The valving 
figure 13. This particular 
was designed so that 1 to 16 
the head could be operated, 
particular device was for 
purposes, it had to operate 
50,000 psi. The mode of operation 
this particular system follows: 



developed a se- 
milling type road 
system is shown in 
arrangement 
segments in 
Since this 
development 
at up to 
of 



84 



O 



o 



o 



o 



o 



o 



o 



o 




FIGURE 13. - Selecting valving arrangement. 



1. Pressure is applied to all the 
valves. In this condition all the valves 
are closed. 

2. The desired segments in the valve 
are opened to tank. This activates the 
valves in that segment. 

3. As the cutting head is rotated, the 
valves get pressure as they leave the 
sections that close the flow to the noz- 
zles. At the same time, as new valves 
enter the drained section, the valves are 
opened. 

The fluid from each on-off valve is 
plumbed through the drive shaft to the 
nozzles in the appropriate quadrant. 
This is shown schematically in figure 14. 




FIGURE 14. - Nozzle plumbing. 



85 




The actual hardware has been built 
and successfully tested. The equipment 
is shown in figure 15. This segmental 
control system is not practical in an 
actual machine because of the flexibility 
requirement . In an actual machine a far 
simpler, smaller, and more maintainable 
system can be developed. 

A second approach to selective assist 
is to mount control valves directly in 
the cutter. Using such a device, the 
jet is activated when the cutter is 
loaded and shuts off when the load is 
removed. The valve shown schematically 
in figure 16 is opened by a fixed travel 
of the cutter. The jet in this par- 
ticular case is an integral part of the 
cutter. 

Both these control systems offer an ef- 
ficient method of achieving the advan- 
tages of water-jet assist while minimiz- 
ing the power requirements and operator 
safety and visibility. 



FIGURE 15. - Actual hardware of phasing system. 

High-pressure water 



passage in bit 
to nozzle 



Force 




W 

^-Trigger 
V/ plate 



Gap, closed by 
bit travel 

Cutting drum 



High-pressure 
water passage 

in drum 

FIGURE 16. - Pick-activated valve. 



86 



CONCLUSIONS 



Water jets used either alone, in com- 
bination with mechanical cutting, or with 
an entrained abrasive can effectively be 
used in a mining operation. The reduc- 
tion in cutting force, dust generation, 
and machine vibration is beneficial for 
continuous cutting operations. The abil- 
ity to slot rock can lead to improved 
blasting and ground control. Because of 
the varying methods of applying water 
jet, there is no good method of applica- 
tion. To use this technique it is im- 
portant to examine the entire mining sys- 
tem. This allows for the opimum use of 
improved equipment or technique for a 
specific operation. 

Once the area of application is identi- 
fied from a system study, a parametric 



evaluation is required to determine the 
best operation conditions for the jet to 
achieve the desired performance. It is 
important to understand that no one oper- 
ating condition meets all applications. 
To take full advantage of water jet at 
minimum energy consumption, it is neces- 
sary to deliver the jet power with mini- 
mum losses. In the case of many mining 
machines sequencing valves must be used 
to eliminate waste. 

Water-jet technology is new to the min- 
ing industry. The techniques and equip- 
ment need to go from the laboratory into 
the field. After experience is gained 
with the equipment , optimization should 
become a simpler process. 



tVU.S. GPO: 1985-505-019/20,102 



INT.-BU.OF MINES, PGH., PA. 28087 



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